A group of B-site ordered double perovskites with a general formula A₂BB&’O₆ are known to exhibit interesting characteristics that are not directly derived from the corresponding simple perovskites, ABO₃ and AB&’O₃. In the case of La₂NiMnO₆ double perovskite, for example, the predominant Ni²⁺-O^2--Mn⁴⁺ ordered structure is ferromagnetic, resulting in a large saturation magnetic moment of 2.5mu;B/B-site and a high Curie temperature of 280K [1]. Moreover, according to Raman and magnetic measurements, La₂NiMnO₆ crystals show strong spin-phonon coupling [2]. This behavior is likely to be related to the observation of magneto-capacitance in La₂NiMnO₆ crystals [1]. In our study, we have investigated the polar state in tensile strained La₂NiMnO₆ films grown on SrTiO₃(001) substrates by using pyroelectric measurements [3] and scanning nonlinear dielectric microscopy (SNDM) [4].
La₂NiMnO₆ films were grown on Nb:SrTiO₃(001) (Nb: 0.05wt%) substrates by pulsed laser deposition [5]. For electric contacts, 100-nm thick Pd top electrodes were deposited on film surfaces by e-beam evaporation. X-ray diffraction scans show that the La₂NiMnO₆ films were epitaxially grown along the c-axis, and the B-site Ni and Mn ions were ordered along the [111] direction. Magnetic characterizations by SQUID gave a saturation magnetization of 2 mu;B/B-site with a Curie temperature of 280K. The presence of a polar state in the ferromagnetic La₂NiMnO₆ films was characterized by pyroelectric current measurements [3]. Systematic modulation of Pd/La₂NiMnO₆/Nb:SrTiO₃ capacitor temperature was used to detect a pyroelectric current, revealing a polar state in the La₂NiMnO₆ films. The effective pyroelectric constant at 140K was 34.3nC/cm²K. This value is comparable to the well-known large pyroelectric coefficient of PbTiO₃/MgO films. A polar state in ferromagnetic La₂NiMnO₆ films was also confirmed by SNDM measurements at room temperature.
[1] N. S. Rogado, et al. Adv. Mater. 17, 2225 (2005)
[2] M. N. Iliev, et al. Appl. Phys. Lett. 90, 151914 (2007)
[3] R. Takahashi, et al. Phys. Rev. B, 86, 144105 (2012)
[4] Y. Cho, et al. Rev. Sci. Instrum. 67, 2297 (1996)
[5] M. Kitamura, et al. Appl. Phys. Lett. 94, 262503 (2009)

The multiferroic perovskite BiFeO₃ has recently become the subject of intense investigation due to its intriguing coexistence of structural polymorphs in highly-strained epitaxial films that show remarkable stripe-like domain patterns[1,2]. The ferroelectric polarization of the strain-stabilized, highly distorted phase has been predicted and experimentally confirmed to reach a value higher than any other perovskite (150 mu;C/cm³)[3-5], all while being an environmentally benign lead-free material. Using synchrotron microdiffraction and piezoresponse force microscopy (PFM) we study how the ferroelectric switching affects the structural properties of both rhombohedral-like (R&’) and tetragonal-like (T&’) BiFeO₃ thin films, both grown on La_0.7Sr_0.3MnO₃ bottom electrodes onto LaAlO₃ substrates. While the T&’ BiFeO₃ is usually obtained on LaAlO₃ substrate, we grew the R&’ BiFeO₃ on the same substrate via (partial) structural relaxation in the La_0.7Sr_0.3MnO₃ film. We find that the as-grown polarization of these two samples points in the opposite direction with respect to each other. Synchrotron microdiffraction results show that switching the polarization away from the as-grown preferred state leads to a clear reduction of the film&’s apparent crystallinity in both films. From in-plane PFM measurements we can directly link this deterioration in crystallinity to a clear reduction in the average size of the monoclinic domains. The reduction in domain size is fully reversible, hence when the polarization is switched back to the as-grown direction the original domain size (and thus the crystallinity) is fully restored. We discuss these results in the context of a strong “built-in” polarization preference of which the direction is determined during nucleation and growth of the film.
Research supported by the U. S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Science and Engineering Division and conducted in part at ORNL&’s Center for Nanophase Materials Sciences and ANL&’s Advanced Photon Source, both supported by the Scientific User Facilities Division of DOE-BES.
[1] C. Beekman et al., Adv. Mater. 25, 5561 (2013)
[2] R. J. Zeches et al., Science 326, 977 (2009)
[3] A. J. Hatt et al., Phys. Rev. B 81, 54109 (2010).
[4] M. D. Rossell et al., Phys. Rev. Lett. 108, 047601 (2012).
[5] J. X. Zhang et al., Phys. Rev. Lett. 107, 147602 (2011).

Ferroelectricity in low dimensions promises lots of scientific and technological benefits. Despite the immense practical potential, however, ferroelectricity tends to be destabilized as the dimension decreases. Critical to these research fields, therefore, is to understand and overcome the size effect in ferroelectricity, which has motivated a flurry of theoretical and experimental efforts. While our understanding still continues to evolve, the suppression of ferroelectricity in a nanoscale system was always inevitable, impeding the broad application of low-dimensional ferroelectricity. In this presentation, we suggest a new strategy to create and enhance ferroelectricity dramatically in low dimensions. Guided by phase-field simulations, we propose that the interfacial charge compensation in ultrathin dielectric films can facilitate a switchable and stable polarization from preexisting polar nanoregions, which are otherwise randomly oriented with little macroscopic polarization. Our strategy challenges the long-standing notion that ferroelectricity is inevitably suppressed in a reduced nanoscale dimension. Our idea was substantiated in a strain-free SrTiO₃ film that was originally non-ferroelectric in bulk, but surprisingly showed room-temperature ferroelectricity for ultrathin thickness below ~10 nm. Detailed studies, using electrical measurement, piezoresponse force microscopy, and transmission electron microscopy, have been carried out to explore the ferroelectricity of ultrathin SrTiO₃ films. Our results shed new light on the low-dimensional ferroelectricity, and give a foundation for exploring nanoelectronics coupled to ferroelectricity.

Increased demand on materials for critical applications requires that we manipulate the structure of materials to meet performance goals. Perovskite type materials with strongly ionic bonds and strong structure dependent properties are most suited to this structural engineering of properties. The mineral perovskite, CaTiO₃, is the archetype of this class of materials and shows dielectric behavior very similar to SrTiO₃. CaTiO₃&’s dielectric constant monotonically increases from room temperature down to ~5 K where it saturates indicating that it is also an incipient ferroelectric. [2] It has been previously demonstrated that ferroelectricity can be induced in thin films of incipient ferroelectrics through epitaxial strain, as shown in SrTiO₃.[1] CaTiO₃ has a similar structure to SrTiO₃, but with an additional octahedral tilt symmetry (a^-a^-c⁺) element that greatly increases the possible distortions under epitaxial strain. Recent theoretical calculations have indicated that CaTiO₃ can become ferroelectric under large biaxial tensile strains of ~1.5%.[3] This induced ferroelectricity has been confirmed through the growth of thin CaTiO₃ films on NdGaO₃ and La_0.18Sr_0.82Al_0.59Ta_0.41O₃ (LSAT) substrates. In this work we demonstrate that, on both substrates, CaTiO₃ exhibits ferroelectricity with an in-plane strain of ~1%. Despite the small strain difference of ~0.1% between the films, CaTiO₃ films grown on LSAT exhibit a much higher transition temperature near 140 K, while it is ~70K for films grown on NdGaO₃ with a reduced polarization. This difference in ferroelectric properties is attributed to the resulting symmetry of the CaTiO₃ grown on the different substrates. (101) NdGaO₃ substrates have a rectangular in^-plane growth net that allows for the bulk-like a^-b⁺c^- tilt system to be maintained, though with a reduced c^- tilt. While the (001) LSAT substrates have a square in^-plane growth net that changes the tilt structure of the CaTiO₃ to a (110) twinned a⁺b⁺c⁰ tilt system. This shows that the ferroelectric state of the CaTiO₃ is dependent on the biaxial strain state in the system and on the octahedral rotations. The details of this interrelation between octahedral tilting and the ferroelectric properties of CaTiO₃ will be discussed.

* This work was conducted at Center for Nanophase Materials Sciences, at Oak Ridge National Lab and the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory, which are sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. DOE.

References
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[2] V.V. Lemanova, et. al., Solid State Com. 110611 (1999).
[3] C. J. Eklund et al. Phys. Rev. B 79, 220101 (2009)

Oxide heterostructure have been a prolific playground for both theoretician and experimentalist to explore emergent phenomena at interfaces. Recently a set of crystal chemistry rules have been defined allowing the design of improper ferroelectrics using Pnma perovskite superlattices [1]. In an effort to extend this set of rules to make them more attractive for experimentalist, a general symmetry argumentation backed up with computational prediction we focused on orthoferrite LnFeO₃/Ln&’FeO₃ heterostructure. The oxide heterostructure [(YFeO₃)₅(LaFeO₃)₅]₄₀ which is magnetically ordered and piezoelectric at room temperature has been constructed from two weak ferromagnetic AFeO₃ perovskites with different A cations using RHEED-monitored pulsed laser deposition. The growth symmetry-imposed polarisation is robust against A site interdiffusion between the two layers which produces a sinusoidally modulated occupancy that retains the coupling of translational and point symmetries required to produce a polar structure. The polarity of the heterostructure is shown by second harmonic generation and piezoforce microscopy measurements.
[1]J.M. Rondinelli, C.J. Fennie, Adv. Mater, 24, 1961 (2012)

As ferroelectric is miniaturized, it faces extreme conditions such as enormous depolarization field E_d and surface effects, of which intrinsic properties are unclarified. P_S-field-effect (field effect by spontaneous polarization P_S) may exist in such condition, relaxing the restrictions by E_d. However, it is usually regarded as extrinsic and is neglected in theories of nano-ferroelectrics and domains.
This is reasonable, because reported P_S-field-effect may be due to (1) defects/impurities, especially, oxygen vacancies, and (2) electron/hole injection, which are examined in this talk. For example, resistance switching in Schottky ^[1-3], pn and tunnel ^[4] diodes using ferroelectrics including nano-scale ones ^[5] is often attributed to (1)(2) or P_S. Similar switching in ordinary insulators (R-RAM), attributed to (1)(2), indicates that (1)(2) are unavoidable in 2-terminal structures. Indeed, the mechanism, P_S ^[1,2] or injection/oxygen,^[3] has been controversial until now. Here, rigorous treatment of current-voltage characteristics is crucial, and those assigned to P_S ^[1] are precisely explained by injection.^[6]
These difficulties (1)(2) exist also in P_S-field-effect on the conductance perpendicular to P_S, i.e., transconductance in 3-terminal. It is noted that the mechanisms (1)(2) exists in MOSFET, the most well-controlled clean system. Indeed, the mechanisms (1)(2) have been unavoidable since the proposals of P_S-field-effect transconductance in 1957,^[7] and transconductance modulation with retention mainly by P_S were achieved after introduction of perovskite-oxide semiconductors.^[8-10]
For these reasons, this talk examines extrinsic effects (1)(2) including P_S-induced modulation doping. Nonetheless, it reports convincing evidences that self-P_S-field-effect ^[11-14] exists in ferroelectric chemically same as that in air. Moreover, although imperfections cannot be reduced be below thermodynamic limit in O₂ (samples contain impurities and defects of a few ppm), the analyses show that this effect is intrinsic and exists in ideal ferroelectrics without imperfections. These conclusions are drawn from transconductance in combination with nanoscopic measurements in UHV. To this end, the choice and preparation of samples are the most essential: BaTiO₃ single crystals are used.
^[1] T. Choi et al., Science324, 63(2009). ^[2] P. Blom et al., PRL73, 2107(1994). ^[3] Y.W. et al.,APL66, 28(1995); Physica C235, 739(1994). ^[4] Y.W., PRB57, R5563(1998); JAP94, 7187(2003). ^[5] Y.W., APL72, 2415(1998). ^[6] Y.W., PRB81, 195210(2010). ^[7] C. Seager et al., Integrated Ferroelectr.6, 47 (1995). ^[8] Y.W., U.S. Patent 5418389(May, 1995). ^[9] Y.W. APL66, 1770(April,1995); Ext. Abst. SSDM&’94, 784 (1994). ^[10] C. Ahn et al., Science269, 373 (July,1995). ^[11] M. Krocirc;mar and C. Fu, PRB 68, 115404(2003). ^[12] R. Cohen, Ferroelectr. 194, 323(1997). ^[13] Y.W. et al, Jpn. J. Appl. Phys. 36, 6162(1997); PRB 789 (1998). ^[14] Y.W. et al., PRL 86, 332(2001).

The electric-field control of data stored in magnetic units offers clear-cut advantages in terms of low-energy consumption and fast information processing. These features prefigure a promising alternative to the nowadays conventional electronics, and foretell a future technology in which nonvolatile memory and data processing elements can be dynamically controlled by external fields. In composite magnetoelectric systems, applied electric fields can modulate the magnetic properties either by mechanical means - via the concerted action of piezoelectricity and magnetostriction effects- or by purely electrostatic effects. The thorough development of such novel electronics demands a complete understanding of the dynamics of the cross-coupling and its response mapped at the microscopic scale. Yet, present experimental approaches to these problems are hampered by the extreme difficulty of simultaneous direct access to the magnetism and ferroelectricity in magnetoelectric devices.
Here we present a comprehensive experimental methodology that exploits optics as a probe of the magnetic and ferroelectric properties. More specifically, we used the effects that ferroelectricity and magnetism exert on the light polarization, i.e., birefringence induced by electro-optic and magneto-optic effects, respectively. The analysis was performed at room temperature in a Pt(10 nm)/BaTiO₃ (120 nm)/La_2/3Sr_1/3MnO₃ (15 nm) trilayer grown on a (001)-oriented SrTiO₃. By analyzing the evolution of light polarization under applied electric fields, we have quantified the magnetoelectric coupling, i.e., we have measured how the magnetization of the La_2/3Sr_1/3MnO₃layer was changed under the application of an electric bias. Additionally, using advanced optics, we reached the diffraction-limited lateral resolution, allowing access to the spatial distribution of the magnetoelectric coupling strength. A stunningly large coupling was found, in which the magnetization was modulated by up to about 50% when electric fields up to 8 volts were applied, with local variations of up to 10% around the average value. The origin of such a large effect is discussed in terms of electric-field modulation of electronic phase inhomogeneities in the La_2/3Sr_1/3MnO₃ film. The observed non-uniform distribution of the magnetoelectric coupling was also correlated to spatial variations of ferroelectricity and strain that were quantified by optical means. These results demonstrate the outstanding potential of optical microscopy to image simultaneously ferroelectricity and magnetism. In particular, the possibility of probing the magnetoelectric coupling deeply buried at interfaces makes this a unique technique, providing novel clues for the dynamic analysis of magnetoelectric coupling in multiferroic systems.

Controlling magnetism by electric fields is a key issue for the future development of low-power spintronics [1]. Progress has been made in the electrical control of magnetic anisotropy [2], domain structure [3,4], spin polarization [5,6] or critical temperatures [7-9]. However, the ability to turn on and off robust ferromagnetism at room temperature and above has remained elusive.
Here we will present a new approach for the electrical control of magnetic and spintronic properties based on the combination of ferroelectric materials with magnetic transition-metal alloys. Experimental results based on X-ray diffraction and various magnetometry techniques will be presented, demonstrating a giant, low-voltage control of magnetism, just above room temperature. The data will be interpreted in the light of first-principles calculations in terms of both strain and field-effect. Our results correspond to a magnetoelectric coupling larger than previous reports by at least one order of magnitude and open new perspectives for the use of ferroelectrics in magnetic storage and spintronics [10].
[1] C. Chappert et al, Nature Mater. 6, 813-823 (2007).
[2] M. Weiler et al, New J. Phys. 11, 013021 (2009).
[3] T. H. E. Lahtinen et al, Sci. Rep. 2, 258 (2012).
[4] M. Ghidini et al, Nature Commun. 4, 1421-1427 (2013).
[5] V. Garcia et al, Science 327, 1106-1110 (2010).
[6] D. Pantel et al, Nature Mater. 11, 289-93 (2012).
[7] D. Chiba et al, Nature Mater. 10, 853-6 (2011).
[8] M. Kawaguchi et al, Appl. Phys. Express 5, 063007 (2012).
[9] H. J. A. Molegraaf et al, Adv. Mater. 21, 3470-3474 (2009).
[10] M. Bibes, Nature Mater. 11, 354-357 (2012)

Epitaxial strain can stabilize new matter phases in thin films and is thus a degree of freedom to increase functionality. In particular, strain-induced ferroelectricity in a magnetically ordered compound enables multiferroic materials which are promising towards novel data processing devices. Aiming for such a strain-induced multiferroic, 20 nm thick films of antiferromagnetic SrMnO3 were grown epitaxially by pulsed laser deposition on different substrates, thus under different strain states. High resolution X-Ray diffraction and transmission electron microscopy were carried out to confirm the crystalline quality of the tetragonal films. Using optical second harmonic generation (SHG), we demonstrate the emergence of a polar state for the films grown under tensile strain confirming theoretical predictions. Furthermore, the combination of SHG and scanning probe microscopy reveals intriguing domain patterns that provide further insight to the nanoscale physics of the new polar phase of SrMnO3.

While ferroelectric materials are normally considered as insulators, semiconducting ferroelectrics have been known for a long time. [1] The coexistence of the ferroelectric phase and conductivity is interesting because such a conducting bistable material introduces new functionalities. We explore the effect of charge carrier doping on ferroelectricity using density functional calculations and phenomenological modeling. By considering a prototypical ferroelectric material, BaTiO3, we demonstrate that ferroelectric displacements are sustained up to the critical concentration of about 0.1 electron per unit cell volume. [2] Our investigations show that the ferroelectric instability requires only a short-range portion of the Coulomb force with an interaction range of the order of the lattice constant. Further, we explore the effect that the switchable polarization of n-doped BaTiO3 (n-BTO) has on the electronic properties of the SrRuO3/n-BTO (001) interface. [3] Ferroelectric polarization controls the accumulation or depletion of electron charge at the interface, and the associated bending of the n-BTO conduction band determines the transport regime across the interface. We find that the interface exhibits a Schottky tunnel barrier for one polarization orientation, whereas an Ohmic contact is present for the opposite polarization orientation, leading to a five orders of magnitude change in the interface resistance associated with polarization reversal. In addition to the resistive switching behavior, we show that interfaces between doped ferroelectrics and ferromagnetic oxide metals may efficiently serve for controlling the spin polarization of carriers injected into the conducting ferroelectrics. In particular, for the SrRuO3/n-BTO interface we predict that ferroelectric polarization switching leads to a sizable change in the transport spin polarization. [4] These results provide a new insight into the origin of ferroelectricity in displacive ferroelectrics and open opportunities for using doped ferroelectrics in novel nanoelectronic and spintronic devices.
[1] V. Fridkin, Ferroelectric Semiconductors (Consultants Bureau, New York, 1980).
[2] Y. Wang, X. Liu, J. D. Burton, S. S. Jaswal, and E. Y. Tsymbal, Ferroelectric instability under screened Coulomb interactions, Physical Review Letters 109, 247601 (2012).
[3] X. Liu, Y. Wang, J. D. Burton, and E. Y. Tsymbal, Polarization controlled Ohmic to Schottky transition at a metal/ferroelectric interface, Physical Review B 88, 165139 (2013).
[4] X. Liu, J. D. Burton, and E. Y. Tsymbal, unpublished.

In our work, we focus on the heteroepitaxial growth of BiFeO3 single crystal thin films on SrTiO3 substrates by pulsed laser deposition. We demonstrate that the initial BiFeO3 growth is very sensitive to the terminating layer of the substrate. Singly TiO2-terminated substrates can be switched to SrO termination by interval deposition with the assistance of Reflective High Energy Electron Diffraction (RHEED) technique. BiFeO3 thin films grown on SrO and TiO2 singly-terminated substrates show different morphologies as well as different ferroelectric domains. This indicates that the sticking of bismuth oxide species on TiO2-terminated surfaces is different than that of iron oxide on SrO terminated surfaces. As the initial growth plays a crucial role in the rest of the growth, it leads to either a step flow or rough growth. This subsequently results in an atomically flat-step by step film surface or a rougher surface with square like features respectively. When grown in step flow regime and depending on the substrate miscut, it is possible to suppress certain types of unit cell structures that can end up with well-ordered domain structures. As the domain and the domain walls are becoming important elements in oxide electronic applications, having an ordered domain pattern makes it easier to address these features.
Keeping in mind that the interfaces become more influential as the film gets thinner, we emphasize the importance of interface engineering for highly controlled multiferroic devices. We claim that, in BiFeO3 thin film growth, the atomic terminating layer of the substrate should be taken into account as well, next to other factors that are known to influence the ferroelectric domain formation, such as screening charges, lattice mismatch or substrate miscut.

Ferroelectric materials are used in a wide array of applications from energy conversion to data storage to actuation and beyond. Recent work in this area has focused, in part, on the identification of Pb-free, high-performance, and high-temperature compatible materials. Large strain-induced enhancements in the ferroelectric Curie temperature (T_C), exceeding several hundreds of degrees, that can be acheived in compressively strained BaTiO₃ (bulk T_C~120°C) has been a significant advancement in this direction. However, there exists limitations to the growth of coherently strained BaTiO₃ thin films, with regards to the magnitude of epitaxial strain as well as the critical film thickness, beyond which a relaxation towards a zero strain state via the introduction of dislocation occurs. In this work, we introduce the possibility of anisotropic lattice deformations from defect complexes as a new route towards engineering strain in these materials. We employ epitaxial strain to control of the electric- and elastic- dipole moments of engineered defect complexes, to systematically enhance T_C to >800°C.
Epitaxial thin films of BaTiO₃ have been grown on a range of substrates including GdScO₃ (110) using pulsed laser deposition. We have employed the laser-fluence as a variable to control the density of growth-induced defect dipoles in these films. As a part of this study, we will first present detailed X-ray diffraction studies and ferroelectric characterization of epitaxial BaTiO₃ films grown on a range of substrates producing both compressive and tensile strains, that reveal an epitaxial strain-driven alignment of growth induced defect-dipole complexes. The electric moment associated with these defect dipoles give rise to local built-in fields that pin the ferroelectric polarization locally, giving rise to the observed shifted and pinched loops. Likewise, elastic dipole moment of these aligned defect dipoles result in an anisotropic deformation of the lattice that manifest in the form of films with largerly expanded out-of-plane lattice parameters. In the remainder of the presentation, we will elucidate the role of the elastic- and electric- moments of these aligned defect dipoles towards stabilizing the tetragonal ferroelectric state of BaTiO₃ . While it has been shown that epitaxial strain alone can enhance the T_C of BaTiO₃ films on GdScO₃ (110) substrates to ~400°C, we demonstrate the ability to controllably enhance the T_C to temperatures exceeding 800°C by tuning the density of aligned defect dipoles in these films. These enhancements are achieved without degradation of the polarization or leakage properties. This work highlights the potential of defect-engineering (coupled with strain-engineering) as a possible route to the enhancement of ferroelectric properties and performance.

Interfaces in complex oxide materials have attracted attentions because of their versatile low dimensional functionalities. Strain engineered epitaxial films of bismuth ferrite provide a polymorphic phase interface between two competing ferroelectric phases. The interface is explored by position-sensitive angle-resolved piezoresponse force microscopy in conjunction with electron holography. We have found that regularly ordered dipole charged domain walls emerge at the interfaces due to flexoelectric polarization arising from giant strain gradient. In addition, we will present our recent observation of anisotropic photocurrent enhancement at the interface. These findings provide new insights into interfacial optoelectronic functionalities.

In strongly correlated electron systems, a modicum of variation of several physical properties - lattice, carrier density, or electronic structure could cause significant phenomena, such as metal-insulator transition, high temperature superconducting, colossal magnetoresistance, and then offer crucial opportunities for developing new electronic devices. Here we demonstrate an anomalous superconducting, which shows large in-plane anisotropy, through fabricating heteroepitaxial structures composing of YBa2Cu3O7not;-σ with periodic BiFeO3 domain structure. Our primary results indicate that a strong in-plane anisotropic superconductivity on the hetero-structures of high temperature superconductor, YBa2Cu3O7not;-σ, and multiferroics, BiFeO3, with periodical domain structures. We also show that these of long-term ferroelectric domain structures suppress the superconductivity in one direction which is perpendicular to domain walls. When the current is parallel to the domain walls, the TC is about 82 K. However, when the current goes perpendicularly to the domain walls, the sample shows non-superconducting behavior. Two possible interpretations are discussed: either the magnetic field, which is created by the uncompensated spin in the domain walls of BiFeO3, or the second phase segregation in YBa2Cu3O7not;-σalong the direction of periodic domain walls. In sum, we provide a path way to manipulate high temperature superconductivity by nano-scale domain wall engineering, and the in-plane anisotropic superconducting could intrigue to a plentitude of studies and applications.

The phase-field model of ferroelectric domains is extended to include the contribution of flexoelectricity by introducing strain gradient - polarization coupling. It is shown that even the classic Ising 180 degree ferroelectric domain wall, long-believed to be charge-neutral, contains both Bloch and Néel type polarization components, and is thus charged. It is demonstrated that the additional polarization components are induced by the stress inhomogeneity around the domain wall through the flexoelectric effect. The magnitudes of the Bloch and Néel components are determined by the competing depolarization and flexoelectric fields with the polarization component parallel to the domain wall as high as several mu;C/cm² and bound charge at the wall as high as 10⁶ C/m³. The application of phase-field model to modeling the flexoelectric effect on domain structures and switching will be briefly discussed. The results suggest the important role of flexoelectricity in determining the properties of a ferroelectric and the possibility to manipulate them through domain wall engineering.

Ferroelectric materials have coupled mechanical, electrical, and thermal responses making them ideal materials for use in applications ranging from transducers, to non-volatile memories, to pyroelectric detectors, and beyond. The prototypical ferroelectric PbZr1-xTixO3 (PZT 1-x:x) family of materials have been widely studied in this context due to their high dielectric constant, low coercive field, and high Curie temperatures. Researchers have exerted control over the properties of these materials by engineering domain structures, chemical composition, epitaxial strain, and a number of other variables. In this presentation, we explore the use of frequency-, AC excitation bias-, and DC background bias-dependent dielectric studies to extract the intrinsic and extrinsic (reversible and irreversible domain wall) contributions to the dielectric permittivity of PZT thin films with strain-engineered domain structures. Single layer PZT 20:80, PZT 52:48, and PZT 80:20 as well as compositionally graded PZT thin films were grown on SrRuO3 bottom electrodes on a range of substrates (SrTiO3 (001), GdScO3 (GSO) (110), and NdScO3 (110)) using pulsed-laser deposition. Asymmetric reciprocal space mapping reveals that the PZT 20:80 and certain compositionally graded films are coherently strained while the remainder of the films studied were partially or fully relaxed. Band-excitation piezoresponse force microscopy images are provided for all samples to explore the evolution of domain structures. Subsequent studies of electronic, dielectric, and ferroelectric properties using symmetric capacitor structures reveal low leakage and excellent ferroelectric properties even at measurement frequencies <1Hz. Dielectric measurements taken under high DC bias showed minimal dependence with increasing AC excitation and frequency indicating complete suppression of the domain wall response. In turn, this approach provides a method by which to quench extrinsic contributions to response and measure the intrinsic dielectric permittivity. The intrinsic dielectric response was found to be relative uniform (ε_int~130-180) across all compositions studied, but was significantly suppressed (ε_int~80) in certain compositionally graded films. This reduction in permittivity is thought to be related to the presence of built-in electric fields (arising from flexoelectric effects) which effectively quenches the low-field susceptibility of these samples. At lower background DC-bias fields, higher extrinsic contributions are observed in PZT 52:48 and PZT 80:20, but this extrinsic response is more easily suppressed by DC-bias fields than in PZT 20:80. The implications of these studies for understanding the stiffness of ferroelastic domain walls and domain wall contributions to the dielectric tunability of ferroelectric thin films will be discussed.

The search for ferroelectric materials with high electric-field (dielectric), stress (piezoelectric), and thermal (pyroelectric) susceptibilities has garnered much attention in recent years as these materials serve as the foundation for modern memory, sensor and actuator, and thermal imaging and infrared detector systems. To ultimately control the nature of such responses, it is imperative that we understand the various contributions to the susceptibility in these complex materials. While contributions to ferroelectric properties from the intrinsic response of the polarization within a domain and so-called extrinsic contributions from the motion of domain walls have been widely studied, the non-motional or stationary contribution from the material within the finite width of the domain wall itself has been particularly difficult to understand. Here we will discuss the direct measurement of the stationary domain wall contribution to dielectric susceptibility in nanodomain PbZr0.2Ti0.8O3 thin films. In this work, we use a combination of theoretical and experimental approaches to investigate the effect of 90° domain walls on the dielectric permittivity of (001)-, (101)-, and (111)-oriented PbZr0.2Ti0.8O3 thin films. Using phenomenological Ginzburg-Landau-Devonshire (GLD) models we calculate the intrinsic and motional extrinsic contributions to permittivity for these different film orientations. Thin-film epitaxy is employed to synthesize highly-controlled model versions of these films. The choice of these three orientations enables us to tune the density of domain walls and thus the relative magnitude of the stationary domain wall contribution to permittivity. In particular, we observe that in (111)-oriented films we can produce high-density, nanotwinned domain structures made entirely of 90° domain walls that experience no extrinsic (motional) domain wall contribution to susceptibilities. Using detailed piezoresponse force microscopy studies we have directly measured the density of domain walls in the materials and can use this to probe the stationary contribution of domain walls. Subsequent comparison of the measured dielectric permittivity and the GLD models reveals a large enhancement of the permittivity that is consistent with a large stationary domain wall contribution. From these measurements we find that the stationary domain wall contribution to the overall permittivity is 6-77.5-times larger than the intrinsic response. These observations provide the first quantitative measurement of the stationary contribution of domain walls and reveal that the large responses at the domain wall can be utilized to improve the overall dielectric susceptibilities.

Multiferroic BiFeO$_3$ (BFO) thin films were grown by pulsed laser deposition on polished spark plasma sintered LaAlO$_3$ (LAO) polycrystalline substrates. Both polycrystalline LAO substrates and BFO films were locally characterized using electron backscattering diffraction (EBSD), which confirmed the high-quality local epitaxial growth on each substrate grain. Piezoforce microscopy (PFM) analysis performed on the corresponding grains show how the persistent ferroelectric behaviour is affected by the polycrystalline nature of the substrate. This high-throughput synthesis process opens the routes towards wide survey of electronic properties as a function of crystalline orientation in complex oxide thin film synthesis.

TbMnO3 (TMO) is a multiferroic material showing antiferromagnetic (AFM) ordering below 42 K and a spin spiral below 27 K that induces ferroelectricity [1]. Strain engineering has been used to modify the ferroic properties of TMO, and epitaxial growth on (100)-SrTiO3 has been proven to induce ferromagnetic (FM) behavior with magnetic moments up to 1.5 mu;B/f.u. at 15 K. The onset of FM has been found inconsistent with homogeneous magnetism, but related to the formation of structural domain walls (DW). In fact, magnetization scales with the DW density [2].
Atomic resolution aberration-corrected Scanning Transmission Electron Microscopy (STEM) combined with Electron-Energy Loss Spectroscopy (EELS) is performed to analyze the domain nanostructure of TMO thin films grown on (100) SrTiO3 in a probe corrected FEI Titan 60-300. High-angle annular dark field (HAADF) imaging evidences the abundance of DWs characterized by the presence of spatially ordered Tb-deficient planar defect accompanied by strong lattice distortions. Plane view observation evidences that DWs of this type are formed along the pseudocubic (100) directions perpendicular to the substrate. Atomic-resolution STEM-EELS chemical mapping demonstrates that Tb-deficient positions are in fact almost entirely occupied by Mn atoms. Analysis of the high energy resolution EELS fine structure of O K and Mn L2,3 edges reveals that the overall consequence is the local decrease of the Mn oxidation state below the nominal value of bulk TMO. This suggests that the Tb-by-Mn substitution is associated to the local presence of oxygen vacancies together with the drastic change of the local environment of Mn atoms in the Tb positions with respect to the “bulk” Mn. DFT+U as well as embedded cluster calculations have been performed to model the magnetic interactions in the DW, suggesting that the presence of Mn atoms in Tb positions introduces local magnetic frustration between the antiferromagnetically coupled Mn3+ ions, which might be the origin of the DW FM observed in this material.
[1] Y. Kimura et al., Nature 426, 55-58 (2003).
[2] C. J. M. Daumont et al.,J. of Phys.: Cond. Matter 21, 182001 (2009).

In ferroelastic thin films, the formation of crystallographic twins provides an additional degree of freedom to control the film crystal structure and it allows coherent growth of relatively thick films on substrates with large mismatch. Using different growth kinetics, differently oriented twins can form to minimize the epitaxial strain, giving rise to different lattice parameters for the same film /substrate combination and the same thickness. That is, the final structure of the strained films does depend on the kind of twins present. At the walls between twins, in addition to breaking of translational symmetry, that already allows different functionalities than those of the domains, also the strain profile shows singular behavior. I will show that the need for a domain wall to accommodate large amounts of strains while keeping the epitaxy, can give rise to changes in the chemistry and lead to novel phases. I will also show that it is the superb control of the formation of nanodomains and, hence, of the domain wall density, what makes these novel phases useful.

Interfacial magnetoelectric coupling for electrically altering the magnetization of ferromagnetic electrodes is a viable path to achieve the electrical writing of the magnetic information in spintronic devices. Exploiting the piezoelectric behavior of a ferroelectric material (FE) in contact with a ferromagnetic (FM) thin film, the electric control of the magnetic anisotropies can be achieved. However strain-mediated methods seems hardly suitable for integration in spintronic devices, where the piezoelectric activity of a FE layer would be inhibited by the growth on a substrate. This is the reason why there is a growing interest towards “purely electric” magnetoelectric effects. For the paradigmatic Fe/BaTiO3 (BTO) system, sizable changes of the interfacial Fe magnetic moment upon reversal of the dielectric polarization of BTO have been predicted. Nevertheless, so far a clear understanding of the basic physical mechanisms leading to such a macroscopic effect is still lacking.
In this paper we demonstrate that the effect of reversing the BTO polarization is not a modest change of the interfacial Fe magnetic moment but the full switching of the interface magnetic ordering, eventually leading to the cancellation of the average Fe interface magnetization. By XMCD measurements on patterned Co(6ML)/Fe(2ML)/BTO/LSMO micro-capacitors, we found that, at RT, the average magnetization of the FeOx interface layer disappears for the dielectric polarization vector of BTO pointing outwards from the FeOx layer. LSDA+U calculations show, indeed, that the switching from the ferromagnetic to the antiferromagnetic ordering within the interface FeOx layer can be driven by the BTO polarization reversal. A “giant” interface inverse magnetoelectric effect results, which can be fruitfully exploited in devices such as tunneling junctions, where just the interface magnetization determines the magnetoresistance.
In view of the exploitation of magnetoelectric effects in a wider class of devices, where the entire magnetization of a magnetic layer should be switched, some recent results on the purely electric control of the magnetic anisotropy in FeCoB thin films on BTO will be finally presented.
[1] S. Brivio, D. Petti, R. Bertacco and J.C. Cezar, Appl. Phys. Lett. 98, 092505 (2011).
[2] C-G. Duan, S. S. Jaswal and E. Y. Tsymbal, Phys. Rev. Lett. 97, 047201 (2006).

We used a complement of photoemission and cathodoluminescence techniques to measure formation of the BaTiO₃(BTO) on SrTiO₃(STO) heterojunction band offset grown monolayer by monolayer by molecular beam epitaxy (MBE). X-ray photoemission spectroscopy (XPS) provided core level and valence band edge energies to monitor the valence band offset in-situ as the first few crystalline BTO monolayers formed on the STO substrate. Ultraviolet photoemission spectroscopy (UPS) measured Fermi level positions within the band gap, work functions and ionization potentials of the growing BTO film. Depth-resolved cathodoluminescence spectroscopy (DRCLS) measured energies and densities of interface states at the buried heterojunction. Kraut-based XPS heterojunction band offsets provided evidence for SrTiO₃/BaTiO₃(STO/BTO) heterojunction linearity, i.e., commutativity and transitivity. In contrast, UPS and XPS revealed a large dipole associated either with local charge transfer or strain-induced polarization within the BTO epilayer.

Use of ferroelectric domain walls for applications is an attractive prospect. Domain walls can have properties not found in the bulk allowing additional functionality. The 1D nature of a ferroelectric domain wall could be exploited to create devices with dimensions on the order of a single unit cell. Intensive research on domain wall conductivity in BiFeO3 is ongoing since the first report in 2009 [1]. Here we report on the evolution of the domain wall conductivity with domain wall length in an epitaxial grown BiFeO3 film grown on self-assembled SrRuO3 nanowires using an ordered mixed terminated DyScO3 substrate as a growth template [2]. The SrRuO3 nanowires (5 nm high, 100 nm wide separated by 200 nm) run across the substrate and are contacted at the sample edge creating alternating insulating/conducting surfaces. Using PFM/cAFM the conductivity of the domain walls is investigated. Conductivity is only observed in domain walls that intersect with an underlying SrRuO3 nanowire. By studying a domain wall that runs across two nanowires the dependence of the conductivity on domain wall length is determined. Previously this length dependence could only be determined by growing samples of different thicknesses which is a time consuming process allowing only a few length scales to be investigate making trends difficult to identify. Here we observe the current through the domain wall decreases rapidly as the domain wall runs off of the conducting nanowires becoming insulating in the region between the nanowires.
B.S., A ten E. and G.K. thank M2i and FOM for financial support grant no. M62.2.08SDMP24.
[1] Seidel J, et. al. Nat. Mat. 2009, 8, 229
[2] Kuiper et al., MRS Communications, Doi:10.1557/mrc.2011.8

Artificial multiferroic heterostructures consisting of materials such as ferromagnetic manganite and ferroelectric titanate layers have shown structural, electronic and magnetic changes in the ferromagnetic layer induced by the ferroelectric layer.[1] Such heterostructures can provide a viable route to room temperature magneto-electric coupling in nanostructured systems.[2] In this work, we show that the orientation of the ferroelectric axis in domains of (011)-oriented Pb(Mg,Nb,Ti)O₃ substrates provides large and reversible room-temperature changes to the magnetic anisotropy of an (La,Sr)MnO₃ thin film epitaxially grown on the substrate. Four-circle X-ray diffraction confirms the highly strained nature of the (La,Sr)MnO₃ film deposited on the Pb(Mg,Nb,Ti)O₃ substrate. Electric field pulse sequences of less than 5 kV/cm can switch ferroelectric domains from an out-of-plane to an in-plane configuration, and there is a corresponding change in magnetic anisotropy from a two-fold in-plane easy axis to four-fold in-plane easy axes. This change in easy axis symmetry is related to the anisotropic strain modulation when rotating the ferroelectric axis from out-of-plane to in-plane, and is stable at zero electric field. We directly illustrate the change between four-fold and two-fold magnetic easy axis configurations in the ferromagnetic domains of the (La,Sr)MnO₃ film by imaging with X-ray photoemission electron microscopy under electric and magnetic field pulse sequences. Furthermore, polarization-dependent X-ray magnetic circular dichroism spectroscopy hysteresis loops measured in zero electric field confirm the reversible and bistable nature of this effect from a highly anisotropic two-fold symmetry to a more isotropic symmetry with a 5 times reduction in hard-axis anisotropy field. Thus, an electric field pulse can be used to ‘set&’ and ‘reset&’ the magnetic anisotropy modulation in the film, showing a promising route towards electric-field assisted magnetic switching of nanoscale magnetic structures at room temperature.
[1] C. A. F. Vaz, J. Hoffman, C. Ahn, and R. Ramesh, Adv. Mater 26-27 2900 (2010).
[2] M. Buzzi, R. V. Chopdekar, J. L. Hockel, A. Bur, T. Wu, N. Pilet, P. Warnicke, G. P. Carman, L. J. Heyderman, and F. Nolting, Phys. Rev. Lett. 111, 027204 (2013).

The coexistence of magnetic and electric order in multiferroics and the resulting magnetoelectric coupling have triggered an immense research interest. The most prominent mechanisms promoting magnetic and ferroelectric order, however, tend to be mutually exclusive. As a result, multiferroics are an inherent source of "unusual" ferroelectricity. In many cases the ferroelectric state is improper, i.e., induced by the ordering of a different parameter like magnetism or strain. This secondary nature can lead to properties not normally found in ferroelectrics. In my talk I will discuss consequences for the ferroelectric domain walls of various multiferroics. In geometric ferroelectrics like hexagonal YMnO₃ the walls exhibit anisotropic conductance and can therefore be regarded as "tunable oxide interfaces" whose properties can be controlled post-growth by shifting the domain walls. In magnetically induced ferroelectrics like MnWO₄ the electric polarization within the wall is expected to rotate instead of passing through zero, as in conventional displacive ferroelectrics, with consequences for the distribution of the domains. In strain-induced ferroelectrics like SrMnO₃ the domain walls can act as insulating boundaries to the conducting domains which therefore acts as nano-capacitors.

The electromechanical properties of piezoelectric and ferroelectric materials can often be enhanced by positioning them at a transition in the phase diagram where two phases are simultaneously accessible, i.e. a morphotropic phase boundary (MPB). BiFeO₃, well known for exhibiting simultaneous antiferromagnetic as and ferroelectric order, has been found to possess a rhombohedral (R) / orthorhombic (O) MPB via substitutional doping of Sm or La and a rhombohedral/tetragonal MPB via application of biaxial strain through thin film epitaxy.
In this work we characterize the structure of coexisting O and R phases in a La_xBi_1-xFeO₃ thin film near the MPB using transmission electron microscopy (TEM). A mixture of the bulk undoped rhombohedral phase and an orthorhombic phase consistent with a PbZrO₃ type structure is observed. Electron diffraction from TEM confirms a quadrupling of the psuedocubic unit cell and is used to show the distribution of the orthorhombic phase within a cross section of the BiFeO₃ film. Orthorhombic domains of two orthogonal orientations are present. These domains are relatively large, often greater than 10 nm in width, with a triangle shape consisting of one vertical and one inclined phase boundary. The inclined boundary forms along one of the (101) planes depending upon the orientation of the orthorhombic phase, indicative of a preferred low-energy boundary configuration. The atomic structure of the O/R boundary is analyzed using aberration corrected scanning transmission microscopy. The preferred phase boundary plane is found to lie parallel to the direction of antiferrodistive A-site displacements of the orthorhombic phase as well as the ferroelectric A-site displacements of the rhombohedral phase. Furthermore, the boundary is located at a position in the orthorhombic unit cell such that the A-site displacements of the rhombohedral phase are closest to those which would have been present in next missing orthorhombic atomic layer. In this manner the large A-site distortions of the two respective phases are accommodated with minimal distortion along the phase boundary plane resulting in a preferred boundary which is nearly atomically sharp.

Doped manganites have emerged as a cornerstone of research in the past two decades, due to an extraordinarily rich spectrum of phenomena, including half-metallicity [1], giant magnetoresistance [2], charge ordering [3], and magneto-elastic coupling [4]. In thin films, the bulk properties can be heavily influenced by the properties of the surface layer and possible reconstructions of surface adatoms, which have been known to e.g. induce metal-insulator transitions [5]. However, very few attempts have focused on obtaining atomic resolution of the as-grown surfaces of these complex oxide thin films, due to the difficulty in preparing clean surfaces, and even fewer attempts have been made at manipulating surfaces with the Scanning Tunneling Microscopy (STM) tips to determine the links between physical properties and local atomic ordering. Here, we report atomic manipulation with STM on the surfaces of 25 unit-cell thick La_5/8Ca_3/8MnO₃ (LCMO) grown on TiO₂-terminated SrTiO₃ (STO) substrates. We demonstrate that by applying triangular first-order reversal curve (FORC) waveforms of increasing amplitude to STM tips in-situ, it is possible from both A and B terminations to individually extract single units, form vacancies, remove units from layers below, rearrange atoms in the surrounding lattice, and therefore cause reactions to occur at the atomic level. Interestingly, experiments reveal that little activity is induced at negative biases, and that most of the reactions occur at about ~+2.3V, coinciding with a current spike or hysteresis loop opening in the FORC curves. These experiments point to the possibility of STM to manipulate atoms on the surfaces of manganites, opening up further avenues of research into fundamental physical properties defined at atomic scales.
References
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2 M. Uehara, S. Mori, C. H. Chen, and S. W. Cheong, Nature 399, 560 (1999).
3 Y. Murakami, H. Kawada, H. Kawata, M. Tanaka, T. Arima, Y. Moritomo, and Y. Tokura, Phys. Rev. Lett. 80, 1932 (1998).
4 S. Lee, A. Pirogov, M. Kang, K.-H. Jang, M. Yonemura, T. Kamiyama, S. W. Cheong, F. Gozzo, N. Shin, H. Kimura, Y. Noda, and J. G. Park, Nature 451, 805 (2008).
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This research was sponsored by the Division of Materials Sciences and Engineering (RKV, AT, SVK) and by the Scientific User Facilities Division (APB) of BES, DOE. Research was conducted at the CNMS, which is sponsored at ORNL by the Scientific User Facilities Division, BES, DOE.

Complex oxides displaying ferroelectric and/or multiferroic behavior are of high fundamental and applied interest. In this work, for the first time, we show that it is possible to achieve polar order in a superlattice made up of two non-polar oxides by means of oxygen vacancy ordering. Using scanning transmission electron microscopy imaging, we show polar displacement of magnetic Fe ions in a superlattice of (LaFeO₃)₂/(SrFeO₃) grown on a SrTiO₃ substrate. Using density functional theory calculations, we systematically study the effect of epitaxial strain, octahedral rotations and surface terminations in the superlattice and find them to have negligible effect on the antiferroelectric displacements of the Fe ions lying in between SrO and LaO layers of the superlattice (i.e., within La_0.5Sr_0.5FeO₃ unit cells). Introduction of oxygen vacancies, on the other hand, triggers a polar displacement of the Fe ions. We confirm this important result using electron energy loss spectroscopy, which shows oxygen vacancy ordering in the region where polar displacements are observed and an absence of vacancy ordering outside of that area. Overall, our results open up a new pathway to design new ferroelectrics and multiferroics.
Acknowledgements: This research was supported by the Materials Sciences and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy (DOE) and DOE grant DE-FG02-09ER46554. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. DOE under Contract No.DE-AC02-05CH11231.

It is well known that tailoring the electronic properties of an oxide is possible by manipulating its processing and structure. Recent results in cuprate films suggest unique properties are also possible in these systems by layering different constituents together into heterostructures and exploiting interface effects.¹ Here, we create “intrinsic” cuprate heterostructures through the strategic ordering of the A-site dopant cations (La³⁺, Sr²⁺) within the single layered perovskite material, LaSrCuO₄. This (A³⁺, A²⁺) cation order creates local electric fields within the material perpendicular to the copper oxide sheets. These induce the other ions in the system (O^2-, Cu³⁺) to respond by moving to screen these local electric fields. These movements in turn modify the Cu-O bond lengths and angles within the structure, potentially resulting in modifications to the electronic properties. For example, the length of the Cu-O_apical bond has been shown to greatly impact the superconducting TC of cuprates.^2,3 This new processing route has the potential to lead to new structures and electronic properties in the cuprates.
Thin film samples of LaSrCuO₄ are grown on isostructural LaSrAlO₄ substrates by oxide Molecular Beam Epitaxy (MBE). The AO layers are either grown as (La,Sr)O for the random alloy control films or LaO and SrO are grown individually in order to achieve a selected pattern of A-site cations. By interleaving full layers of LaO⁽⁺¹⁾ (L), SrO⁽⁰⁾ (S), and CuO₂^(-1) (C) in varying order, a variety of chemical patterns within the unit cell can be created along the growth direction: [LCS/LCS], [SCL/SCL], [LCS/SCL], and [LCL/SCS]. It is important to note that this strategic cation ordering creates these interfaces within each unit cell (designated by brackets), thus maintaining the overall composition of the film. Each film is compositionally identical; however, the local electrostatics differ markedlly. Samples are monitored during growth using Reflective High-Energy Electron Diffraction (RHEED). After growth, the sample structure and surface roughness are characterized using X-ray reflectivity (XRR), X-ray diffraction (XRD), and Atomic Force Microscopy (AFM). Electrical properties (ρ vs. T) are measured using 4-pt probe electrical transport measurements. Through XRR simulations, we assess the surface roughness, film thickness, and superlattice density profile as a function of depth of the samples. We find that the ability to stabilize the desired cation order differs between ordering patterns. Finally, we discuss how processing factors (growth temperature and deposition rate) affect the ability to stabilize the desired cation ordering patterns.
1. Logvenov, G., et al. Science 326, 699 (2009).
2. Eisaki, H., et al. Phys. Rev. B 69, 064512 (2004).
3. Slezak, J. A., et al. Proc. Natl. Acad. Sci. USA 105, 3203 (2008).

Framework structures (cf. perovskites) built by linked octahedral units, may exhibit specific deformation mechanisms under interfacial stress which are governed by the relative strength between the bond angles bridging adjacent units and the bonds within the units. When interfacial polarity adds to the interfacial stress, spin, charge and orbital degrees of freedom, characterizing strongly correlated oxides, may interact with octahedral tilting and elastic strain. This behavior brings forth a radically new misfit strain relaxation scenario in heteroepitaxial growth with dramatic consequences on the functional properties of the films. Here we take the strongly correlated perovskite oxide La2/3Sr1/3MnO3 as a model system to explore lattice distortions and the electronic structure under tensile and compressive misfit stresses and the presence of a polar discontinuity, using x-ray diffraction, linear dichroism, and electron microscopy. In both cases an interfacial layer governed by the electronic interaction with the substrate is formed, while above a critical thickness compressive stresses favor plastic relaxation. The thickness evolution of the magnetic and transport properties of the films are revisited on the basis of these findings.

Shape-induced anisotropy in ferromagnets offers a powerful tool for magnetic device engineering. This phenomenon is well understood from the principles of magnetostatics. A similar anisotropy was theoretically predicted for compensated antiferromagnets (AFM) with a strong magnetoelastic coupling. Recent experiments by our group have uncovered distinct shape effects in the domain pattern and orientation of the Néel vector for AFM nanostructures defined in epitaxial LaFeO3 thin films. Using element specific x-ray spectromicroscopy, we investigate how the AFM domain patterns depend on size, shape, and orientation with respect to the crystalline axes and demonstrate how nanoscale engineering may serve to stabilize a monodomain AFM state. Our findings shed light on the mechanisms governing the domain formation in AFM thin film nanostructures. The capability to control the AFM domain state in patterned thin films holds promise for enhanced performance of nanoscale magnetic devices relying on the coupling between an antiferromagnet and a ferromagnet.

The strong structure-property relationship in perovskite oxides enables controlled design of functionalities. One possible route towards that end is the synthesis of epitaxial thin films with different crystallographic orientations. In (111)-oriented epitaxial systems, the symmetry is hexagonal, with triangular facets of cations and oxygen atoms at the surface. We have recently found a coupling between the underlying crystalline symmetry and the magnetic domain structure in (111)-oriented ferromagnetic La0.7Sr0.3MnO3/ SrTiO3 thin films. Here, we report on the magnetic order of antiferromagnetic LaFeO3, grown epitaxially by pulsed laser deposition on SrTiO3(111), and discuss how the (111)-orientation affects the magnetic coupling to an adjacent ferromagnetic oxide. X-ray diffraction and transmission electron microscopy show that the films are structurally monodomain, while x-ray linear dichroism imaging (XMLD-PEEM) unveil a multidomain antiferromagnetic domain pattern, and the relationship between the magnetic easy axes and the underlying quasi-hexagonal lattice will be discussed. In LaFeO3, being a G-type antiferromagnet, the (111) surface is spin polarized in contrast to the fully compensated (001) surface. A detailed study of the x-ray magnetic dichroism (XMCD/XMLD) in epitaxial bilayers of (111)-oriented LaFeO3/ La0.7Sr0.3MnO3 will serve to shed light on how the magnetically polar interface affects the interlayer exchange coupling. The microscopic magnetic response will be correlated to macroscopic VSM studies.

High quality La2/3Sr2/3MnO3 (LSMO) thin films prepared by sputtering are studied by using Conducting Scanning Force and Magnetic Force Microscopy. LSMO thin film surface consists of one unit cell steps separating atomically flat terraces, with a measured surface roughness on the terraces lower than 0.3Å. In spite of the extremely flat surface morphology observed, simultaneously acquired topography and current maps revealed a considerably intensity along lines superimposed on a quite uniform current background. These enhanced conducting lines are not randomly distributed but form domains with periodic arrays with perpendicular orientations, which coexisted either on the same or different terraces. The periodicity of the lines within each domain coincides with the lateral size of each crystallographic twin as observed by XRD and SEM, thus evidencing that the observed lines correspond to the precise position of the twin boundaries. Current versus voltage characteristics obtained at specific surface points indicate an important enhancement of the electronic response at the twin boundaries locations. The absolute values of the measured current for a given voltage may differ by one order of magnitude. A clear magnetic signature is also detected at the twin boundaries locations. The origin of this large difference is still under consideration, but preliminary analysis seems to indicate that an increase in the density of states at the boundaries might have an important contribution.

We have grown VO2/NiO epitaxial thin film heterostructures by pulsed laser deposition technique where integration with Si(001) substrates was successfully achieved by cubic-yttria-stabilized zirconia (YSZ) buffer layer. The in-plane and out-of-plane orientations of the thin films were studied by theta;/2theta; and phi; scanning XRD, respectively. The epitaxial relationship across the YSZ/Si(001) interface was shown to be {001}YSZ||{001}Si and <100>YSZ||<100>Si. In the case of YSZ/NiO interface, the epitaxial relationship was expressed as (001)YSZ||(111)NiO and [010]YSZ||[110]NiO. Finally, the epitaxial relationship at the NiO/VO2 interface was explained as (010)VO2||(111)NiO and [100]VO2||[110]NiO. The epitaxial growth and the proposed atomic arrangements were confirmed by TEM imaging and diffraction pattern. The semiconductor to metal transition behavior of the films was also investigated where a near bulk and sharp transition was observed at about 341.5 K. These desirable electrical characteristics are attributed to fully relaxation of the misfit strain along the c-axis of VO2.
VO2(010)/NiO(111) epitaxial heterostructures were integrated with Si(100) substrates using a cubic yttria-stabilized zirconia (c-YSZ) buffer. The epitaxial alignment across the interfaces was determined to be VO2(010)||NiO(111)||c-YSZ(001)||Si(001) and VO2[100]||NiO<110>||c-YSZ<100>||Si<100>. The samples were subsequently treated by a single shot of a nanosecond KrF excimer laser. Pristine as-deposited film showed diamagnetic behavior, while laser annealed sample exhibited ferromagnetic behavior. The population of majority charge carriers (e-) and electrical conductivity increased by about two orders of magnitude following laser annealing. These observations are attributed to the introduction of oxygen vacancies into the VO2 thin films and the formation of V3+ defects.

Study on exchange coupled magnetic nanocomposites is being aggressively pursued to enhance energy product of magnetic nanocomposite systems. Beside metal-metal systems such as SmCo5-FeNi, Nd2Fe14B/Fe, SmCo5/Nd2Fe14B system, recently efforts are made towards enhancing energy product of oxide-oxide based hard-soft system. The oxide based nanocomposites are attractive in terms of ease of production, non-corrosive, light weight, and thus are very energy efficient system. Exchange-coupled oxide based nanocomposites such as SrFe12O19/NiZnFe2O4, SrFe12O19/CoFe2O4 etc have been achieved by systematically varying soft-ferrite content. In the present study, instead of varying soft ferrite content, we have varied magnetization of the soft component in the nanocomposite. Magnetic nanocomposite (hard) SrFe12O19-(soft) La(1-x)SrxMnO3 powders were synthesized via autocombustion method in 4:1 weight ratio. The magnetization of the soft phase was La1-xSrxMnO3 varied by systematically varying x value, thus controlling the saturation magnetization of the soft phase in the composite. Structural and morphological characterizations were performed via x-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD analysis shows presence of pure phase components in the nanocomposite. TEM images show presence of needle shape Sr-Ferrite particles. The room temperature magnetic measurements revealed the higher Mr/Ms ratio for the nanocomposites than that for the single phase SrFe12O19 which proves the existence of the intergrain exchange coupling between hard and soft magnetic phases with the exchange spring behavior. The highest Mr/Ms ratio of 0.588 was obtained in the composite consisting SrFe12O19- La0.25Sr0.75MnO3. This sample also exhibited 72% increase in the coercivity (Hc ~ 6.26 kOe) as compared to pure SrFe12O19 (Hc~3.63kOe). The variations of the reduced remanence (Mr/Ms) with changing magnetic softness of the soft phase could be also explained by the role of the exchange and dipolar interactions in tuning the magnetic properties of the nanocomposites. The study presents for the first time demonstrated synthesis of hard-soft exchange couple nanocomposites by controlling the “magnetic softness” of a soft phase instead of more traditional approach of varying the concentration of the soft phase.

We have studied a laser MBE-based atomic layer-by-layer growth technique for oxide thin films and heterostructures that allows us to assemble the unit cells one atomic layer at a time and to achieve the desired stoichiometry. In the growth of SrTiO3 from the separate SrO and TiO2 targets, we monitored the reflection high-energy electron diffraction (RHEED) intensity to control the flux for each atomic layer. The phases of the specular and diffraction spot intensities as well as that of the Kikuchi lines were investigated. UV Raman spectroscopy was used to probe the symmetry breaking due to the cation off-stoichiometry. Similar stoichiometry control as shown by reactive MBE has been demonstrated. This method has been applied to the deposition of LaAlO3 and LaNiO3 thin films.

The effect of the structural transition in BaTiO₃ (BTO) on the magnetic properties of the magnetostrictive material CoFe₂O₄ (CFO) is presented. Epitaxial thin films of CFO were grown on SrTiO₃ (100) (STO) substrates with a La_0.7Sr_0.3MnO₂ (LSMO) film as the bottom electrode to form the multiferrroic heterostructure STO/LSMO/BTO/CFO. A novel dual laser ablation technique (DLA) that combines the outputs of a pulsed CO₂ and a KrF excimer laser was used for film growth. The resulting high plasma temperature and percentage of ionization of the ablated plasma facilitates the growth of defect-free uniform films with high crystallinity, which is essential for forming heterostructures with film thicknesses in the nanoscale.
Epitaxial film growth was confirmed using X-ray diffraction data (XRD), and atomic force microscopy images showed smooth surfaces with low roughness values. Scanning electron microscopy images revealed that, compared to films grown using single laser ablation, DLA yielded films with virtually no particulate matter. Films with only the LSMO and CFO layers showed perpendicular magnetic anisotropy with the easy axis along the film plane. Compared to saturation magnetization (M_s) of 230 emu/cm³ with coercive field (H_c) of 0.1 kOe of LSMO thin films, M_s of 366 emu/cm³ and H_c of 2.6 kOe were measured for LSMO/CFO bilayers at 300 K. Magnetic measurements performed on LSMO/BTO/CFO and LSMO/CFO films at 278 K and 183 K corresponding to the tetragonal to monoclinic and monoclinic to rhombohedral transitions of BTO respectively, showed large jumps in the magnetization as a function of temperature in films with BTO. Further investigations of the magnetic properties were performed by incorporating multiple layers of BTO. The ferroelectric properties of BTO, with and without the CFO layer, were also measured, utilizing top and bottom LSMO layers as electrodes.

Magnetic oxides are known to host a number of interesting physical phenomena, for which electron-electron interactions and electron-lattice coupling are the fundamental driving forces. One particularly striking manifestation of such intricate balance of interactions is the observation of magnetoresistance that, in some specific materials like manganite oxides, can be of several orders of magnitude, the so-called colossal magnetoresistance (CMR). A huge amount of work has been devoted to unveil the fundamental mechanisms leading to magnetoresistive behavior in magnetic oxides, and a great consensus is met in that polaron physics and more specifically, electron-phonon coupling, is at the basis of such phenomena. Yet, a vast majority of studies are focused at the low-frequency or dc-regime, where the electronic transport is analyzed under the application of quasi-static electric fields. It is of not surprise that, by limiting the analysis of the electronic phases at low frequencies, a large amount of rich physics may remain unseen. Although certainly some studies have been directed towards the analysis of the optical conductivity responses in the near-IR and visible, it is also true that no direct link has been established between the intensity of optical responses associated to the suppression of polaron conductivity with magnetic fields and the electron-lattice coupling strength in a variety of magnetic oxides.
Our work bridges up the paucity of this knowledge. We have performed an extensive study of the spectral optical magneto-reflectance of a number of oxides, embracing manganites, magnetite and strontium ruthenate, for which the electron-phonon coupling is modulated from extreme values, from materials that exhibit strong coupling to those which are broad-band metals with weak electron-lattice interactions. Interestingly, we observe that the magneto-reflectance variations are strictly correlated with the electron-phonon interaction strength, so that optics appears as a reliable and powerful technique to explore the physics of correlated oxides and opens the way to new fields of research. For instance, it could pave the way towards scrutinizing the dynamics of polaron conductivity in those systems. Remarkably, our research also envisages the possibility of exploiting optics for probing electron correlations in materials that are responsive to stimuli other than magnetic fields, such as strain in Mott-Hubbard insulators like some titanates and vanadates.

Ternary rare earth oxides are attractive for gate applications due to their high dielectric constants. Ternary rare oxides, in comparison to binary rare earth oxides, have similar or even higher dielectric constants (up to 40), and are much more stable in a chemical environment. When deposited, such oxides can be grown either as polycrystalline or as single crystals, namely, epitaxially on various substrates including III-V semiconductors and perovskites. When grown as thin films, these oxides present interesting properties that depend heavily on their structure and interfaces with the adjacent materials.
Thin films of LaLuO3 were deposited using pulsed laser deposition on (100) Nb doped SrTiO3 under different conditions. The growth conditions enabled to produce well oriented layers of LaLuO3. The orthorhombic lattice of LaLuO3 presents two similar lattice constants (a=5.83A, c=6.02A), while the third is significantly larger (b=8.38A). This low symmetry crystalline structure causes different properties for different directions in the lattice due to dissimilar atomic ordering.
Using transmission electron microscopy and x-ray diffraction in various configurations including pole figure measurements, a complete study of the crystalline structure of the films was investigated. Thus as an example, X-ray diffraction symmetric theta;-2theta; scans showed different patterns for different deposition conditions on the same substrate orientation. The two different patterns obtained demonstrate two epitaxial layers well oriented in the out of plain direction, but with different plains, (110) and (001), parallel to the (100) substrate surface. Therefore we obtain different atomic ordering at the interface that is created between the grown LaLuO3 film and the underlying SrTiO3 substrate. Electrical characterization of the layers was carried out to examine the differences in the dielectric properties that arise from the change in the structure of the layer and of the interface.

LaAlO₃/SrTiO₃ heterointerfaces have displayed intriguing properties such as metallic, insulating, superconducting, and magnetic properties at low temperatures. The deposition conditions of the LaAlO₃ thin films can be tuned to select for these properties. Here we demonstrate that the choice of substrate preparation method also has an effect on the interfacial properties. Using pulsed laser deposition, epitaxial LaAlO₃ thin films were deposited simultaneously on (001)-oriented SrTiO₃ substrates prepared with the well-known buffered hydrofluoric acid (BHF) etching method and the deionized-water (DI-water) leaching method [1]. Metallic samples with n_s > 10¹⁴ cm^-2 displayed little difference in carrier concentrations. However, less metallic samples with n_s < 10¹³ cm^-2 showed an order of magnitude difference in conducting carriers depending on the substrate preparation method at low temperatures, presumably due to additional carriers provided by fluorine ions during substrate preparation. Our results indicate that the choice of substrate preparation method has an impact on the transport properties of LaAlO₃/SrTiO₃ heterointerfaces.
[1] J. G. Connell, B. J. Isaac, G. B. Ekanayake, D. R. Strachan, and S. S. A. Seo, Appl. Phys. Lett., 101, 251607, (2012).

Thin film perovskite-based heterostructures have many attractive properties that often arise from the local atomic structure and chemistry within the material. One way to modify these properties via structure manipulation is by utilizing the relative size of the cations within the structure. These steric packing schemes can have a significant effect on transition metal-oxygen bond lengths and bond angles, in turn directly manipulating the local transition metal crystal field. Here, in order to direct the local transition metal crystal field in new ways, we look to combine this well-known steric tailoring method with a novel method that harnesses local electrostatics. In recent experimental and theoretical work in epitaxial layered nickelate films of LaSrNiO4, similarly-sized La and Sr cations were ordered into different layering patterns within the crystal structure. This resulted in local electric fields that modified the Ni-O bond lengths and angles, and local crystal field.
In order to investigate how this electrostatic cation ordering phenomena can be coupled with large differences in dopant cation size, we investigate the effects of ordering La and Ba cations in LaBaCuO4. By depositing full monolayers of LaO(+1), BaO(0), and CuO2(-1) within the film, but in different ordering patterns, one is able to vary the local electric fields present. These local electric fields influence the lowest energy positions of the other ions in the system—most importantly, the oxygen anions that form an octahedra around the Cu cation. This in turn modifies the Cu-O bond lengths and angles, and can potentially alter the properties within the material.
Thin film samples of LaBaCuO4 are grown on LaSrAlO4 substrates by oxide Molecular Beam Epitaxy (MBE). The AO layers are either grown as (La,Ba)O for the random alloy control films or LaO and BaO are grown individually in order to achieve a selected pattern of A-site cations. By interleaving full layers of LaO(+1) (L), BaO(0) (B), and CuO2(-1) (C) in varying order, a variety of unit cell patterns can be created along the growth direction: [LCB/LCB], [BCL/BCL], [LCB/BCL], and [LCL/BCB]. It is important to note that this strategic cation ordering creates these interfaces within each unit cell (designated by brackets), thus maintaining the overall composition of the film. Each film is compositionally identical; however, the local electrostatics differ markedly. Compositionally identical samples with strategic ordering or random alloying are monitored during growth using Reflective High-Energy Electron Diffraction (RHEED). After growth, the sample structure and surface roughness are characterized using X-ray reflectivity (XRR), X-ray diffraction (XRD), and Atomic Force Microscopy (AFM). Electrical properties (R vs. T) are measured using 4-pt probe electrical transport measurements. Through XRR simulations, we assess the surface roughness, film thickness, and cation order within the samples.

Epitaxial interfaces between perovskite oxides are promising for device applications, exhibiting novel physical properties not found in bulk of the constituent materials. Such systems support many remarkable and potentially useful effects, including superconductivity and magnetism. Perovskite oxides exhibit a strong structure-property coupling, and the properties of the interface can hence be influenced by structural effects. Here we present an ab-initio study comparing how oxygen vacancies affects structural relaxation at the interface. Specifically, we focus on the resulting interface structure a rhombohedral, (La,Sr)MnO₃. , material is deposited on cubic SrTiO₃. Based on density functional theory (DFT) calculations we present how the oxygen vacancy structure induce changes in the octahedral tilting pattern of the deposited materials . We will especially discuss the possibility of ordered vs. unordered vacancy structures, the ordered being ca. 0.2eV/f.u. more stable for the oxygen deficient brownmillerite phase, and possible implications on oxygen vacancy mobility. The theoretical results are compared with experimental transmission electron microscopy data on pulsed laser deposited thin film heterostructures exhibiting oxygen vacancy driven phenomena.

In epitaxial oxides, the substrate surface structures (termination, atomic configuration or stoichiometry) can play a decisive role in determining the electronic properties of the system. Due to the polar nature of specific crystal orientations (like [110] and [111]), a charge imbalance is created at the surface. Surface reconstruction is one possible way in which the crystal surface can become charge neutral. However, in many oxides how the surface reconstructs is widely unknown. Particularly it is unknown how the surface reacts to various pretreatments, and to changing temperature and partial pressures, as occurs under typical growth conditions. In this talk, we will demonstrate in-situ X-ray studies revealing SrTiO3 (111) single crystal surface structures under such circumstances. By combining surface X-ray diffraction and coherent Bragg rod analysis method we have been able to reconstruct the electron density profiles of the bulk crystal as well as the disturbed surface layers with a sub-angstrom precision. Understanding electron density distribution and electrostatic boundary conditions helps to retrieve the physical solution to the surface structures. This analysis has been performed on SrTiO3 (111) substrates that are as received as well as after several different pretreatments In addition to room temperature measurements we have also measured the (111) surface at 550°c in a 10-3 mbar O2 environment. This has enabled us to observe how the polar surface changes near a typical growth condition and how this change may affect interface properties, and the epitaxial growth upon the surface thereafter.

Because SrTiO₃ (STO) is favorably lattice-matched to many other complex oxides, it is a widely-used substrate for epitaxial oxide growth. However, the surfaces of STO crystals, as provided by the manufacturer, do not typically have a unique surface termination and lack the necessary atomically-flat, defect-free surface required for successful oxide epitaxy. Two commonly used methods of preparing atomically-smooth, TiO₂-terminated STO surfaces consist of acid-based etching [1,2]. More recently, in an effort to avoid the safety issues of acidic etchants, it has been shown that various forms of water-leaching are able to provide atomically flat, TiO₂-terminated STO, but its effectiveness compared to these acid-based methods was unkown [3-5]. We employed high-resolution X-ray photoelectron spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES) to study the effectiveness of various surface preparations of Nb-doped STO (001) [6]. Because ARPES is very sensitive to both long-range order, as well as to surface defects, it served as an excellent tool in determining the effectiveness of various preparation methods. It was found that different preparations had a noticeable effect, not only on surface quality, but also on the presence of mid-gap states. ARPES measurements revealed a non-dispersing, mid-gap state ~800 meV above the top of the valence band for samples which underwent etching. This mid-gap state is not present in samples which were simply vacuum-annealed, or thin STO films grown using molecular beam epitaxy (MBE). Theoretical modeling using density functional theory (DFT) suggests that this mid-gap state is not related to the SrO- or TiO₂-termination of surfaces, but rather, is due to a partial hydrogenation of the STO surface that occurs during etching.

[1] M. Kawasaki et al., Science 266, 1540 (1994).
[2] M. Kareev et al., Appl. Phys. Lett. 93, 061909 (2008).
[3] J. E. Boschker and T. Tybell, Appl. Phys. Lett. 100, 151604 (2012).
[4] S. A. Chambers et al., Surf. Sci. 606, 554 (2012).
[5] J. G. Connell et al., Appl. Phys. Lett. 101, 251607 (2012).
[6] R. C. Hatch et al., J. Appl. Phys. 114, 103710 (2013).

Identifying the stable surface at relevant environmental conditions is a prerequisite for a materials utilisation in any given field of application. Ab initio based surface phase diagrams provide such information and have proven to be an important tool in areas such as semiconductor physics or catalysis. They depict the stability of surface phases, typically as a function of the temperature and pressure of a surrounding atmosphere [1]. Extending their applicability to the consideration of an electrochemical environment appears quite desirable, but modelling electrochemical system presents a challenge to first-principles methods.
We have developed a method which links ab initio calculations quite naturally to experimental observables, such as the pH-scale and the electrode potential, which determine and characterise the state of an electrochemical system. Based on the formation energies of ions in solution, it is similar to the approach used in the defect chemistry of semiconductors [2]. We utilise our recently developed method to explore the stability of the polar Zn terminated ZnO(0001) surfaces in an electrochemical environment.
[1] M. Valtiner, M. Todorova, G. Grundmeier, J. Neugebauer, Phys. Rev. Lett. 103, 065502 (2009).
[2] C.G. Van de Walle and J. Neugebauer, J. Appl. Phys. 95, 3851 (2004).

Oxide surfaces are important for many applications, especially for catalysis, electronic devices and energy storage [1,2]. Oxides are also used as insulating layers (e.g., in electronic devices) or corrosion protection layers (e.g., for water splitting), their interface or surface electronic states can significantly influence the related carrier transports. Oxides are characterized by their complexity in crystal structures and the variability of their structures as functions of constitute metal elements. As a result, they often elude the study of trends in their properties. While the scientific community is actively searching for new oxides for different applications, there is an urgent need for understanding general trends in their properties to guide such searches. One example of such general understanding for bulk oxides is the Fermi pinning rule, and the observed difficulty to dope large band gap oxides in p-type due to the appearance of oxygen vacancies [3]. However, there is still a lack of general understanding of the oxide surface electronic states [4], in particular: under which conditions will the surface band gap state appear when there is no ligand passivation? Large volumes of literatures exist for individual oxide surface studies, but little trend and not much systematic knowledge have emerged from such scattered studies. For main group semiconductors, it is generally understood that a surface termination without significant reconstruction or surface ligand passivation will always create inside band gap surface dangling bond states due to the break of surface covalent bonds. An electron counting rule and its relevance to the surface passivation of covalent bonding main group semiconductors has been established [5]. However, oxides can be viewed as between covalent and ionic bonding, and their properties can be very different from that of main group semiconductors. In this communication, through ab initio calculations, we found that, for all the transition metal nonpolar oxide surfaces considered here, none of them has inside band gap states. We will discuss the reasons for this phenomenon, and hypothesize that this could be a general rule for transition metal oxides.
1. E. I. Altman, U. D. Schwarz, Adv. Mater. 2010, 22, 2854.
2. J. Jiang, Y. Li, J. Liu, X. Huang, C. Yuan, X. W. Lou, Adv. Mater. 2012, 24, 5166.
3. S. B. Zhang, S. H. Wei, A. Zunger, Physica B 1999, 273-274, 976.
4. P. A. Cox, Transition Metal Oxides, Clarendon Press, Oxford, UK 1992.
5. G.P. Srivastava, Appl. Surf. Sci. 2006, 252, 7600.

Disappearance of conductance stripes at a vacuum-cleaved SrTiO₃ (001) surface was observed together with an emergence of a new state upon low temperature annealing. This disappearance was found to associate with an evolution from the initial alternating stripes with TiO₂/SrO surface termination into step-terrace structure with mostly TiO₂ termination. During the evolution, an incomplete terrace with holes that are half a unit cell deep were observed. Interestingly, tunneling spectra obtained from the TiO₂-terminated terrace which contains sufficiently low density of the half-step hole reveals the new state around 1.5-1.7 eV above the Fermi level while the spectra of the TiO₂-terminated surface with high density of hole exhibit only a typical state around 2.25 eV. This development of the new state could be associated with different degree of oxygen deficiency and mild surface reconstruction in which both are related to the completeness of the terrace across the surface. On the other hand, the change in surface termination occurs due to the fact that the TiO₂ termination is more thermodynamically favorable during ultra-high vacuum annealing. Consequently, the results of this work presents a step forward towards understanding the origin behind the creation of the conductance stripes and 2D electron gas at the cleaved SrTiO₃ surface.

We have demonstrated the generation of a quasi two dimensional electron gas (q2DEG) at the interface of LaAlO3 and Ru doped SrTiO3. There have been recent reports of ferromagnetism at the LAO/STO interface where it is generally agreed upon that metallicity can be generated [1-4]. In this work the Ru4+ dopants are introduced onto the Ti4+ site of a STO film onto which a LAO capping layer is grown in order to investigate the role of isovalent doping of Ru moment in the conduction channel at the LAO/STO interface. X-ray diffraction indicates epitaxial growth of 2 nm thin films of SrTinot;0.98Ru0.02O3 grown on STO substrates on which a LAO overlayer of 2 to 20 nm was epitaxially grown. X-ray magnetic circular dichroism measurements do not reveal ferromagnetic ordering at either the Ti or Ru L edges down to 10 K. Transport measurements indicate metallic behavior down to 2 K, while resistivity, carrier concentration, and carrier mobility values are within the range of values reported for undoped LAO/STO interfaces. The temperature dependence of the carrier mobility saturates at low temperatures and follows a power law fit for phonon scattering at high temperatures. Mobile carrier concentration at room temperature approaches saturation at around 10-14 cm-2 - only a third of the total carrier concentration predicted from the polar catastrophe model. The limited modification of the low temperature carrier mobility compared to undoped samples further indicates the excellent crystalline quality of the doped STO film in which the q2DEG resides. These transport measurements indicate that scattering in the quasi-two-dimensional electron gas is not significantly altered by the addition of Ru moments in the 2DEG at the LAO/STO interface.
[1] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004).
[2] M. Huijben, G. Rijnders, D. H. Blank, S. Bals, S. van Aert, J. Verbeeck, G. van Tendeloo, A. Brinkman, and H. Hilgenkamp, Nature materials 5 (7), 556 (2006).
[3] J. A. Bert, B. Kalisky, C. Bell, M. Kim, Y. Hikita, and H. Y. Hwang, K. A. Moler, Nature Physics 7, 767 (2011).
[4] L. Li, C. Richter, J. Mannhart, and R. C. Ashoori, Nature Physics 7 (10), 762 (2011).

The quasi-two-dimensional electron gas (Q-2D-EG) that forms at the interface between two perovskite band insulators LaAlO3 and SrTiO3 has stimulated extensive research interest since its discovery in 2004 by Ohtomo et al.[1] The physical origins of the Q-2D-EG formed at the interface have been under intensive debate to date. Several mechanisms have been proposed, such as the polar catastrophe at the polar LaAlO3 /non-polar SrTiO3 interface, structural distortions, oxygen vacancies introduced into the LaAlO3/SrTiO3 heterostructure during the growth of LaAlO3 or preferential cationic intermixing at the interface. Limited reports on quantification of behavior as well as assumptions about the similarity of films in the literature constrained the efforts to understand fundamentals and resulted in such conflicting reports.
Compositional and structural factors affecting electronic properties of two dimensional metallic interfaces are being identified in conjunction with the process conditions. The work presented here focuses on the structure and chemistry of LaAlO3/SrTiO3 hetero-structures produced by PLD at different oxygen partial pressures and plume angles. By coupling several analytical and imaging techniques such as HR-TEM (High Resolution - Transmission Electron Microscopy), Ultra-STEM (Scanning TEM, probe size ~ 0.7 Å) and Energy Loss Spectroscopy (EELS), as well as Rutherford Backscattering Spectrometry (RBS), the dislocation density, strain level and (non-)stoichiometry are established together with the cation intermixing, atomic displacements, dipoles, and Ti valence state across the interface. As a future step, the results presented will be used to describe the quantitative impact of structural and compositional factors on the density and mobility of charge carriers at LaAlO3/SrTiO3 interfaces.
[1] A. Ohtomo, and H.Y. Hwang, “A High-Mobility Electron Gas at the LaAlO3/SrTiO3 Heterointerface,” Letters to Nature, 427, 423 (2004).

TbMnO3/Nb-doped SrTiO3 (TMO/NSTO) heterostructures were fabricated by epitaxial grown TbMnO3 thin films on (001) Nb-1wt% doped SrTiO3 single crystal substrates using RF magnetron sputtering technique. The heterostructure showed normal diodelike behavior above 175 K, and backward diodelike behavior below 175 K. Correspondingly, in case of forward bias, the conduction mechanism is a Schottky-emission-like conduction above 175 K and a space-charge-limited conduction below 175 K, whereas in case of reverse bias it is space-charge-limited conduction in whole temperature range, such different conduction mechanism between forward bias and reverse bias should be caused by the difference of depletion width in the interface between p-type TMO and n-type NSTO under different directional bias. Temperature dependent resistance in both of the forward and reverse bias appeared an abnormal transition around Curie temperature (TC = 28 K), under which the magnetic-electric coupling exists in TMO, displaying correlation between the magnetic properties and the electrical properties.

Novel material properties often emerge when the material is structurally modified compared to its perfect bulk state. Examples of such scenarios include epitaxially strained thin films and crystal defects. CaTiO3(CTO), an incipient ferroelectric material in its bulk state, may show ferroelectric behavior either at ferroelastic domain boundaries [1] or under lattice strain [2]. It is of great interests to see how the strain, crystal defects and other factors interplay at the actual epitaxial CTO films. AC-STEM is a powerful characterization tool to study epitaxial thin film systems, revealing local structural and chemical information at sub-Å level [3].
Two 20 nm CaTiO3 films were grown on LSAT and NGO by pulsed laser deposition. The resultant CTO films showed ferroelectric behavior with Tc near 140 K on LSAT and near 70 K on NGO, and the remnant polarization at 10K of 5 and 2 uC/cm, respectively. These differences cannot be simply explained by a relatively small difference in strain measured by X-ray diffraction (i.e. ~1.2% on LSAT and ~1.1% on NGO).
Our AC-STEM results show that CTO films have a so-called "c-ePbnm" configuration on both substrates, with c-axis of the orthorhombic unit cell lying in the plane of the film. However, two major differences in microstructure were observed. Firstly, the first few nm of CTO on NGO show defect-free epitaxial growth, and after that grains start to develop but the c-axis of CTO remains aligned with the c-axis of NGO, which suggests that only 180o grain boundaries are present in this system. However in CTO films on LSAT, different grains start to develop at the interface and their c-axes have two possible in-plane orientations at 90o to each other, resulting in the formation of both 180o and 90o grain boundaries. Some of these grain boundaries appear to be inclined mixed dislocations, and others appear to be ferroelastic twins. Secondly, we observed a difference in octahedral tilt behavior at the film/substrate interface: CTO films on LSAT have a 5-6 unit cell thick transition region from the untilted LSAT to the tilted CTO, while on NGO both film and substrate have a similar value of tilt therefore there is no such transition zone.
These characterization results suggest that even though the nominal strain of the two films is about the same, the in-plane symmetry of the substrate changes the microstructure in the films. The connection between the microstructural details, substrate strain and connections to the ferroelectric state will be discussed in detail.
[1] S. Van Aert et al., Adv Mater 24 (2012).
[2] C. J. Eklund et al., Phys. Rev. B 79 (2009).
[3] S. J. Pennycook, Ultramicroscopy 123 (2012).
* Research supported by the Materials Science and Engineering Division, U.S. Department of Energy (DOE), and through a user project supported by ORNL&’s Center for Nanophase Materials Sciences, which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. DOE.

Domain wall motion in ferroelectric thin films is crucial to the polarization switching process. In multi-axial ferroelectrics such as Pb(Zr0.2, Ti0.8)O3 (PZT), complete ferroelectric switching often requires motion of ferroelastic domain walls in response to the applied field due to the complex domain patterns in the film. These domain walls are associated with long range elastic stress fields that interact with elastic defects in the thin films, such as misfit and threading dislocations frequently observed to be associated with the domain walls. In this study, phase field modeling is used to investigate the impact of dislocations on domain wall mobility in ferroelectric thin films. We simulate ferroelastic domain wall motion under applied electrical potentials and mechanical loads in epitaxial PZT films in the vicinity of dislocations. Results show 90° ferroelastic domain wall motion was hindered by interactions with the long-range elastic stress fields of dislocations both near the surface of the thin film and throughout the thickness of the film. Both misfit and threading type dislocations were considered and interactions between the dislocations and ferroelastic domains were analyzed. Mechanical and electrostatic states of the thin films around the dislocations were analyzed to understanding the pinning mechanisms. Edge dislocations near the substrate with Burger&’s vectors of ½ a[101] were found to be most effective at pinning domain walls and hindering their motion during switching.

Tuning the magnetic and piezo/ferroelectric properties of bismuth ferrite, BiFeO3 (BFO), by means of chemical substitution or strain engineering has been a subject of great recent interest. In this study, the role of cobalt substitution in the crystal structure and piezo/ferroelectric properties of BFO thin films was studied. With the aid of epitaxial strain, (1-x)BiFeO3 -xBiCoO3 (BFCO) (0 < x < 0.15) thin films, which is obtained only by high-pressure synthesis in a bulk form, were successfully stabilized, and an enhancement in their piezoresponses was observed. This enhancement is explained in terms of a compositionally driven phase transition, the so-called “morphotropic phase boundary”. Moreover, in the BFCO film with x = 0.15, an unconventional strain relaxation behavior, resulting in V-shaped or wide-stripe domains of tetragonal-like phase with a “giant” c/a ratio of approximately 1.25 on SrTiO3 substrates with compressive strain of only around 1.5%, was observed. This observation demonstrates that the crystal structure and piezo/ferroelectric properties of BFO can be finely tuned by a combination of cobalt substitution and epitaxial strain.

Epitaxial constraints from a substrate onto a complex-oxide thin film make it possible to stabilize new crystalline phases and modify existing materials. While earlier studies have focused primarily on the consequences of long-range biaxial compression or tension, recent emphasis has been on the more subtle effects of local distortions, including polarization and octahedral tilts. For ABO₃ perovskites, the coherence of a network of fairly rigid BO₃ octahedra typically extends across the interface, and therefore strongly influences the film material within a few unit cells of this discontinuity. Electron microscopy studies by a number of teams have in fact revealed this continuity for various perovskites, including BiFeO₃ films on SrTiO₃ substrates. Interestingly, when BiFeO₃ is grown on LaAlO₃ substrates (i.e. under much larger compressive strain), the structure changes to one where the atomic arrangement can no longer be described by a network of octahedra but rather one where the Fe ion is essentially 5-fold coordinated. We have used synchrotron x-ray diffraction to probe the unit cell of such highly axial BiFeO₃ (referred to as T&’-BiFeO₃) and show that the systematic presences and absences of half-order Bragg peaks can in fact not be interpreted in the framework of Glazer tilt patterns (contrary to the case of R&’-BiFeO₃). When these films are investigated as a function of film thickness, we observe a transition between different monoclinic symmetries and ultimately to a true tetragonal phase at the lowest thicknesses. These results are compared to data from BiCrO₃, a related perovskite that, on the same LaAlO₃ substrate, crystallizes in a perovskite structure with the typical network of octahedra.
Research supported by the U. S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Science and Engineering Division and conducted in part at the Advanced Photon Source, Argonne National Laboratory (Scientific User Facilities Division, DOE-BES).

Rare earth titanate perovskites RTiO₃ (where R is a trivalent rare earth ion such as Gd, Sm, Pr, hellip;), have a Mott insulating character and rich magnetic phases arising from the coupling of the t_2g orbitals and spin in the 3d¹ state through strong electronic correlations. For smaller rare earths, the compounds are ferromagnetic and insulating, which is very rare in simple perovskites, and thus offer interesting opportunities for oxide spintronics through e.g. spin filtering. Thin films of rare earth titanates may also allow the study of emergent concepts, such as correlated 2D electron gases at their interfaces [1], and their use in Mottronic systems [2, 3]. Here we focus on thin films of ferromagnetic and insulating GdTiO₃. In the bulk, GdTiO₃ crystallizes in an orthorhombic Pbnm structure (with parameters a=5.402 Å, b=5.697 Å, and c=7.68 Å) and presents a strong GdFeO₃-type distortion. Its magnetic properties have been described as a lattice of Ti spins S=1/2 ordered ferromagnetically below 32K and antiferromagnetically coupled to the Gd lattice (spin S=7/2), giving rise to a saturation magnetization of around 6 mu;B per formula unit [4].
We report on the synthesis of epitaxial thin films of GdTiO₃ on SrLaGaO₄ substrates by pulsed laser deposition. Structural characterization by X-ray diffraction show that the GdTiO₃ layers are fully strained and free of impurity phases. Films synthesized under optimum conditions possess a Curie temperature T_C = 31.8 K with a saturation magnetization around 5 mu;B per formula unit at 10K. We propose a possible scenario to account for the small differences between thin film and bulk magnetic properties. Transport measurements reveal that our films are insulating as expected. This study shows that it is possible to stabilize high quality GdTiO₃ thin films which are insulating and ferromagnetic on SrLaGaO₄ substrates by mean of pulsed laser deposition. Finally, first-principles calculations of films and bulk GdTiO₃ are also performed, and will be compared with our measurements.
[1] Pouya Moetakef, Tyler A. Cain, Daniel G. Ouellette, Jack Y. Zhang, Dmitri O. Klenov, Anderson Janotti, Chris G. Van de Walle, Siddharth Rajan, S. James Allen and Susanne Stemmer, Appl. Phys. Lett. 99, 232116, 2011
[2] D. M. Newns, J. A. Misewich, C. C. Tsuei, A Gupta, B. A. Scott, and A. Schrott, Appl. Phys. Lett., 73, 780, 1998
[3] M. Nakano, K. Shibuya, D. Okuyama, T. Hatano, S. Ono, M. Kawasaki, Y. Iwasa and Y. Tokura, Nature 487, 459, 2012
[4] H.D. Zhou, J. B. Goodenough, J. Phys. Condens. Matter 17, 7395-7406, 2005

VO₂ exhibits a metal-insulator transition (MIT) at around 340 K and thus has attracted much attention for its applications such as electrical switches and sensors. In addition to electrical applications, the MIT can be utilized for optical switches because of its significant change in optical properties. It is known that the MIT is also accompanied by the structural change from high-temperature rutile (R) to low-temperature monoclinic (M) phases and thus strains on VO₂ thin films have a significant effect on the MIT. For instance, a tensile-strained VO₂ film on TiO₂ (001) substrate has a reduced MIT temperature (T_MI) of ~300 K. Substrate material choice is therefore of importance in controlling the T_MI.
In this study, we have chosen MgF₂ as a substrate to grow epitaxial VO₂ films. MgF₂ is isostructural with R-phase VO₂ and the lattice mismatch can be minimized to be 1.6 % by choosing the (001) plane. Epitaxial VO₂ thin films were fabricated by pulsed laser deposition. We found that the T_MI of epitaxial VO₂ films on MgF₂ substrates can be controlled by choosing an appropriate substrate temperature during the deposition. X-ray diffraction experiments revealed that the change in the T_MI is attributable to the change in the strain state imposed by their substrates. Raman scattering spectra exhibits several sharp peaks in the M-phase. In the R-phase, on the other hand, no sharp peaks were observed but a broad one was done in the frequency range of 400-600 cm^-1. The Raman scattering study revealed that the peaks originated from V-V vibration modes of a strained VO₂ film shift to higher frequency, compared with those of a relaxed one. This result suggests that the epitaxial strain has an impact on the V-V distance and/or the tilting angle of V-V dimer in the M-phase VO₂ film, which results in the change in the T_MI of the strained film.
This work was supported partly by the Japan Society for the Promotion of Science (JSPS) through the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program), initiated by the Council for Science and Technology Policy (CSTP).

Vanadium sesquioxide (V2O3) is a strongly correlated material that exhibits a metal-insulator-transition (MIT) at low temperatures. The electrical triggering of this transition could result in an exciting new category of applications, such as resistive switching-based memories and field effect transistors. We have fabricated V2O3 microbridges by combining Molecular beam expitaxy growth with UV lithography and dry etching. Resistivity measurements are done as a function of temperature using a four-point configuration. In this way, the MIT is studied in microbridges with different length/width aspect ratios. We found that the size of the MIT is largest for the widest and shortest microbridges. We discuss the influence of device processing in the observed behavior.
We have also measured voltage-current characteristics (VI) at different temperatures across the MIT for different bridge dimensions. We observe a sudden change in the resistance of the device while continuously sweeping the current at intermediate temperatures. At low (insulating state) and high (metallic state) temperatures no switching of the resistance with current sweeps is observed within the range defined by the experimental voltage compliance of 20V.
The observed resistance switch is always a change to a more metallic state and the only way to switch back to an insulating state is by thermal cycling. We have estimated the power transferred to the device by the applied current in order to understand this behavior in terms of local Joule heating. The distribution of size of the resistance jumps and the values of voltage and current at which these jumps occur are studied as a function of width and length of the microbridge.

Complex functional oxides are outstanding materials giving rise to physical phenomena such as high Tc superconductivity, colossal magnetoresistance, ferroelectricity, and more recently to Resistive Switching (RS), where two reversible resistive states can be induced, opening the possibilities for new RRAM based electronics devices. Oxides with metal-insulator transitions (MIT) have a unique opportunity to display area RS effects if this MIT can be controlled by an electric field. In addition, interfaces between different oxides as well as with electrodes are a key point in this phenomenon. The mechanism underlying this phenomenon is still unclear though oxygen vacancies mobility is believed to have a key role. We early proved [1] that reversible transitions from low resistive (LR) to high resistivity (HR) states and even multilevel switching was possible at the nanoscale in La0.7Sr0.3MnO3 films by means of Conductive-Scanning Force Microscopy (C-SFM). Here, we present our studies on a series of different functional oxides heterostructures with different thickness based on manganites (La0.7Sr0.3MnO3), nickelates (LaRO3 with R=Ni, Nd, Sm or mixed compositions) and superconducting cuprates YBa2Cu3O7 grown by chemical solution deposition; in some of them an ultrathin CeO2 (ionic conductor) top heterostructure was deposited by atomic layer deposition (ALD). We have observed reversible RS in all these systems giving rise to a complete writing and erasing process. Transitions can be induced even through thickness (25 nm) envisaging new opportunities. HRSTEM analysis confirms the epitaxial character of all these layers and the strain states. In particular, ultrathin (3-10 nm) LSMO on STO grows in a coherently strained state and a completely relaxed LSMO is obtained on LAO, while a Curie temperature of 350 K is maintained in both cases. The MIT of both heterostructures is however modified by film thickness and strain state. The RS characteristics of these layers and bilayers have been evaluated by C-SFM but also by I-V characteristics using Pt electrodes. STEM, local optical conductivity and PEEM studies on written and erased areas are underway in order to further pinch the mechanism involved in this phenomenon.

With the advancement of microelectromechanical systems (MEMS), capacitive micromachined ultrasonic transducers (cMUTs) have recently been introduced as an alternative to existing bulk piezoelectric technology. To date, cMUTs have demonstrated significant advantages including wide bandwidth and high yield manufacture. However, cMUT&’s electrostatic actuation creates unavoidable limitations including the necessity of high DC bias voltage, non-linear behavior, small gap separation, and pull-in instability. Most importantly, acoustic power output is limited by the geometric constraints including inevitable small gap height.
The piezoelectric micro-machined ultrasonic transducer (pMUT) is a promising alternative to the cMUT since it does not suffer from the limitations related to electrostatic transduction. The piezoelectric response is linear, does not require a DC bias voltage and without a required gap, the pMUT&’s deflection and thus acoustic power output is not as limited. Despite its potential, insufficient modeling of existing pMUTs has produced devices that fall short of modeling predictions, resulting in low electromechanical coupling and reduced bandwidth. We developed a novel green&’s function approach to analytically solve the plate vibration equation for deflection considering an arbitrary number of circular or ring electrodes. This modeling tool opens up new opportunities for understanding PMUTs and implementing an optimal design framework for high-power ultrasound micro-transducers.
After fabrication of the initial design of pMUT, an extensive evaluation of the results was performed to understand pMUT&’s transduction performance. Device deflection under applied AC voltage was measured with white light interferometry, which confirmed a strong agreement between the analytical model and experimental results. For acoustic domain characterization, a test tank with a 3-axis, 0.01mm resolution positioning system was fabricated for beam pattern, transmit and receiving sensitivity, bandwidth, and other acoustic pressure measurements. Testing was performed in an oil bath on individual 100µm diameter circular devices with resonant frequencies of asymp;2.1MHz in oil. As predicted, the measured pressure output is linear with applied AC voltage for moderate values of electric field. Transmit sensitivity as high as 250Pa/V per cell at a range of 5mm and bandwidth of 10-20% were measured.
Preliminary experiments with first generation devices already show potential for improved acoustic power output over cMUT technologies, and linear transduction properties. Substantial modeling and hands-on micro-fabrication and experimental measurement with transduction systems has allowed us to identify a core group of significant design metrics including acoustic power, bandwidth, electromechanical coupling, electrical and mechanical quality factor, transmit and receiving sensitivity that will be considered in future design iterations.

We simulate a nano-scale ferroelectric actuator using a phase field model of material behavior and modify the design to optimize performance. The concept is based on 90 degree switching in barium titanate, producing ferroelectric/ferroelastic strains much greater than those due to the piezoelectric effect alone.
The barium titanate crystal is treated as a 2-dimensional continuum containing domains with different polarized states, subjected to electrical and mechanical boundary conditions. With charge-free surface conditions, the domains form a flux closure with zero net polarization, such as a vortex structure. Application of electric field can then force the polar direction to align with the field. This reorientation also induces mechanical strain. Based on this principle we demonstrate the concept of a ferroelectric nano-actuator, simulating its response with a previously developed 2D-phase field model.
The working principle of actuation is demonstrated using an embedded actuator which is a nano-scale region comprising two electrodes vertically separated by a region of clamped pre-stressed barium titanate. The idea is to apply an electric field causing a net polarization and tensile strain along the field direction, resulting in actuation. On switching off the electric field, the actuator is returned to its initial state by the clamped mechanical boundary conditions. Actuation strains of the order of 0.4% are attained for an embedded actuator with an in-plane pre-strain of 0.82%. The actuator response to uniformly distributed loads opposing its motion is studied.
The design of nano-actuator has been improved to achieve greater actuation displacements by exploiting the concept of bending of a slender beam. The actuator design comprises a slender beam fixed at its ends, with electrode pairs on upper and lower surfaces. The electrode pattern is designed to allow switching between an ‘off&’ state with polarization through the beam thickness, and an ‘on&’ state with polarization aligned along the length of the beam. The longitudinal strain associated with the ‘on&’ state causes a large sideways bending displacement. Our simulations of this actuator indicate actuated displacements of the order of 18% of the device height.
The design parameters of the actuators have been further optimised to enhance the achievable actuation strains. Our initial calculations of the electrical energy required to actuate the embedded actuator have been quantified and the efficiency of the device has been estimated while working against uniform loads. The converse piezoelectric property of barium titanate suggests the possibility of simulating a nano-generator using the phase field model. An external load causing mechanical deformation of the ferroelectric nano-actuators, such as varying the substrate strain, can result in the appearance of charge on the electrodes.

Memristors are continuously tunable resistors that can be used to emulate synapses. Conceptualized in the 1970s, they traditionally operate by voltage-induced displacements of matter. Purely electronic memristors have recently emerged based on well-established physical phenomena with albeit modest resistance changes. We show that voltage-controlled domain configurations in ferroelectric tunnel barriers yield memristive behavior with large resistance variations at 10 ns operation speed [1]. The tunneling current is influenced by the polarization direction [2] enabling a simple non-destructive readout of the polarization state.
Here we study tunnel junctions based on super-tetragonal BiFeO₃ tunnel barriers [3]. They show stable, multistate switching with very high resistance ratios of more than four orders of magnitude. Combined piezoresponse force microscopy (PFM) and electrical measurements provide insight into the switching characteristics in response to trains of nanosecond pulses. We also present recent single shot experiments on the transient switching dynamics.
[1] A. Chanthbouala et al., Nature Nanotechnology 7, 101 (2012); A. Chanthbouala et al., Nature Materials 11, 860 (2012)
[2] E.Y. Tsymbal and H. Kohlstedt, Science 313 (2006)
[3] H. Yamada et al., ACS Nano 7, 5385 (2013)

Currently, large efforts are going on to scale up PZT thin films for MEMS in order to meet highest performance for industrial needs. It this work we report about the microstructural and piezoelectric properties of RF magnetron sputtered PZT (Pb(Zrx,Ti1-x)O3) thin films. PZT is a ferroelectric perovskite which is characterized by a peak in the piezoelectric properties at the so-called morphotropic phase boundary (MPB) which occurs for x=0.53. While its synthesis in bulk ceramic form is fully mastered, still many issues need to be studied in thin films with regard to optimal properties and performance limits. In this work we focused on RF magnetron sputtering. We report about results obtained on 150mm silicon wafers with a commercial sputtering setup capable to reach 1nm/sec of deposition rate with thickness uniformity of better than +/-3% on 150mm wafers and +/-4% on 200mm wafers, respectively. Nominal target composition of x=0.53 leads to films with x=0.540 +/- 0.002. The Pb concentration is stoichiometric within the error of the EDAX method (19+/-2 %). Particularly we show how the texture can be chosen between (100), (111) and “quasi-random “ on fully passivated Pt electrodes, avoiding any perovskite seeding layer. We obtain extraordinary narrow X-ray diffraction peaks with a FWHM of only 0.09° in the {100} theta-2theta experiment. The surface morphology, i.e. roughness and grain shapes, is strongly influenced by the self-bias on the substrate. Stress can be varied between -80 MPa + 50 MPa, which is about 100 MPA less than in sol-gel PZT thin films. The effective transverse piezoelectric coefficient e31,f of a 2.5 µm thick film reached 22 C/m2 in the actuator mode (converse piezoelectric effect), which is a remarkable value for un-doped, pure PZT thin films.

Epitaxial anatase titanium dioxide (TiO₂) attracts increasing attention in electronic applications due to its remarkable physical properties, including room-temperature ferromagnetism and transparent conductivity when chemically doped with transition metal ions [1, 2]. In order to fully utilize these properties, electrostatic carrier control using field-effect transistor (FET) structures is an ideal approach since the carrier density can be tuned continuously without introducing dopants. Up to now, several studies on epitaxial anatase TiO₂ metal-insulator-semiconductor FET (MISFET) were reported [3, 4]. However, MISFET suffers from degradation of the channel mobility due to interface scattering, because the gate electric field pushes the channel carriers toward the gate/channel interface. On the other hand, a metal-semiconductor FET (MESFET) using a Schottky gate is free from interface scattering, because the depletion region separates the channel carriers from the gate/channel interface.
Here we present a depletion-mode epitaxial anatase TiO₂ MESFET using Pt as the Schottky gate electrode. A series of devices were fabricated using pulsed laser deposition to epitaxially deposit one monolayer of LaO on LaAlO₃ (001) substrates with atomic-layer precision, followed by anatase TiO₂ thin films. The LaO monolayer was deposited following a recent study reporting that epitaxial anatase TiO₂ thin films grown on LaO-terminated LaAlO₃ (001) substrates show a high mobility metallic state with outstanding crystalline quality [5]. Excellent rectifying behavior was achieved at the Pt/anatase TiO₂ Schottky junction with a gate leakage as low as 10^-3 A/cm². As a result, the on/off current ratio of epitaxial anatase TiO₂ MESFET exceeded 10⁶ in a voltage range of 3 V at room temperature. These results demonstrate that carriers can be easily tuned in a wide range, and imply that exotic transport properties can be further studied in the same device structure.
[1] Y. Matsumoto et al., Science 291, 854 (2001).
[2] Y. Furubayashi et al., Appl. Phys. Lett. 86, 252101 (2005).
[3] M. Katayama et al., Appl. Phys. Lett. 92, 132107 (2008).
[4] Y. Nagao et al., Appl. Phys. Lett. 97, 172112 (2010).
[5] M. Minohara et al., 2012 MRS Spring Meeting, San Francisco, CA, USA (unpublished).

A fast, low power, transistor-type switching device has been proposed in which piezoelectric and piezoresistive materials are employed in a stacked sandwich structure of nanometer dimension. It has been predicted that piezoelectronic transistors could enable an order of magnitude increase in computer clock speed, while simultaneously reducing the power between 10 and 100 times. Achieving this performance requires high performance, high aspect ratio piezoelectric 70Pb(Mg_1/3Nb_2/3)O₃-30PbTiO₃ (PMN-PT) films. Both oriented and epitaxial PMN-PT films of 0.3 microns in thickness were made by a 2MOE solvent sol-gel route on substrate sizes up to 8” in diameter. These films were strongly {100} oriented, phase pure by XRD with dielectric constants exceeding 1500 and loss tangents of < 0.05. The films showed slim hysteresis loops with remanent polarizations of about 8-12 mu;C/cm² and breakdown field exceeding 1500 kV/cm. Fully clamped films exhibited large signal strain of over 1% with low signal d_33,f of approximately 90 nm/V. It was found that the properties improve significantly on laterally patterning the piezoelectric layer by reactive ion etching; including a large increase in the domain wall contributions to the dielectric response. These results suggest that progressive declamping is more important than etch damage in controlling the observed properties of the scaled ferroelectric films. Finally, incorporation of piezoelectric films into piezoelectronic transistors, using SmSe piezoresistors was demonstrated, yielding functional devices.

BaTiO3 (BTO) is a ferroelectric oxide that can be grown on Si and Ge substrates with the use of an appropriate buffer. The formation and properties of an epitaxial interface between BTO on Ge is the focus of our study. N-type Sb-doped (001)-oriented Ge substrates (0.029-0.054 #8486;-cm) were used in this study. BTO films were grown on Ge using molecular beam epitaxy. An aberration-corrected, high-angle, annular-dark-field (ADF) electron microscopy reveals the atomically sharp interface between the two materials. An x-ray diffraction symmetric 2theta;-omega; scan of a 40-nm film of BTO on germanium reveals an out of plane lattice constant of 3.97 Å. This indicates that the BTO film is a-axis (100) oriented. The valence band offset (VBO) between a thin (five unit cells) BTO layer and Ge was measured using x-ray photoelectron spectroscopy and was found to be 2.7±0.1 eV. Adding the experimentally measured band gaps of Ge and BTO gives us a conduction band offset (CBO) of -0.2±0.1 eV, with the conduction band of Ge being higher than that of BTO. Using density functional theory (DFT) in the local density approximation, we examine multiple interfaces between BTO and Ge, and calculate the band offsets for each of them. We find that the band offset depends strongly on the screening of the interface and the polarization state of BTO. For the interface built using the ADF image, we find that, for in-plane polarized BTO, the VBO is 2.50 eV. This value matches well with the experimentally measured VBO of 2.6 eV. For comparison, using the Schottky limit and experimentally measured electron affinities gives a 2.4 eV VBO. That the offsets are quite close to the Schottky limit suggests a very small shift in the energy due to an interfacial dipole.

The interface properties of heteroepitaxially grown polar materials have received much attention due to the existence of quantum confined 2D carrier gasses (2DCG). Integrating polar oxide materials with wide-bandgap semiconductors offers the possibility of a tunable 2DCG - provided interfaces are smooth and defect densities are kept low. A lattice-matched rocksalt oxide epitaxially grown on GaN is an excellent prototype for more challenging multifunctional oxides and charts a path toward 2D interface conductivity via a polar discontinuity in a chemically and structurally dissimilar system. Unfortunately, preparing oxide-nitride interfaces of sufficient quality to observe a 2DCG is technically daunting. Smooth, commensurate and single domain oxide-nitride epitaxy that also incorporates an intrinsic carrier source to populate the quantum well remains elusive. Recently, however, a new surfactant-assisted MBE growth technique has been used to prepare smooth (111) MgO and CaO rocksalt oxides on GaN. Surfactant-assisted MBE of lattice matched rocksalt oxides is an enabling technology that allows for preparation of ‘semiconductor grade&’ interfaces in heterogeneous systems and provides access to a rich frontier of new applications at the intersection of multifunctional oxides and technologically relevant nitride semiconductors.
In this submission we focus on two prototypical rocksalts, MgCaO (MCO) and CdMgO (CMO) that will allow us to answer questions about the source of carriers in a rocksalt-GaN 2DCG as well as provide insight into the origin of rotation domains commonly observed in oxide-nitride epitaxy. Cd1-xMgxO is a wide-bandgap n-type oxide semiconductor while MgxCa1-xO is an insulator with a bandgap approaching 8 eV. We attempt to encourage an electronic compensation for the interface polarization discontinuity by lowering the free energy of forming a 2D electron gas (2DEG) through doping. CMO is easily doped by elements in the lanthanide series like Dy while adding a few mol percent Cd to insulating MCO may have a similar effect. Growth of lattice matched CMO and MCO on GaN using both Plasma-assisted MBE (PAMBE) and pulsed-laser deposition is demonstrated. Optical characterization of the rocksalt thin films as well as transport properties will be presented. In addition, we will introduce evidence for an epitaxial origin of the two in-plane rotation domains that frustrate the growth of large-area commensurate interfaces. Finally, we will share efforts to control the surface morphology of MOCVD grown GaN substrates by removing the characteristic c/2 bilayer fluctuation. Selected area epitaxial (SAE) regrowth of GaN on low threading dislocation density substrates allows for preparation of single-terrace flat GaN and, combined with lattice matched smooth MCO, near perfect oxide heteroepitaxy. Collectively these techniques open new avenues for integration of oxides on GaN and observation of unique interface phenomena.

During recent years, energy harvesting from vibration and motion sources has attracted much interest. Electrical power generators based on piezoelectric materials were investigated, first as flexible piezoelectric bulk composite materials containing PZT ceramics, and later also as MEMS devices based on thin films, or even nanowires. In case of micro power devices, the main target applications are wireless communication, sensors, and specifically wireless sensors. Average supply powers of 100 µW are sufficient to operate wireless nodes with low duty cycles. There is general agreement on the fact that motions and vibrations constitute the most versatile and ubiquitous ambient energy sources available, if light harvesting is excluded by the application. The progress in piezoelectric thin films and MEMS technology has lead to the development of demonstration devices that show sufficiently large power outputs and voltage levels, i.e. more than several 100 µW/cm2 at over one volt. The standard configuration is to use parallel plate ca-pacitor geometry exploiting the transverse piezoelectric coefficient. It looks evident that Pb(ZrTi)O3 and related ferroelectric materials should be the best choice. However, ferroelectric thin films exhibit high dielectric constants - meaning low voltage output - and usually suffer from limited remanent polarization. Remedies can be found in using interdigitated electrode systems, or imprinted films. Another solution, however, could be to use AlScN thin films. Substituting Al in part by Sc leads to higher e31,f coefficients with only a moderate increase of the dielectric constant.
In this talk, the principles of piezoelectric energy harvesting will be introduced and illustrated with achieved results. Direct strain coupling to moving parts is compared to acceleration coupling by means of inertial masses. Specific thin films issues will be addressed such as material&’s choice, and electrode configurations. The interdigitated electrode system will be discussed in more detail, including growth on insulators, poling, domain issues and performance. The energy figure of merit e*h as derived from measurements is introduced to compare materials and electrode systems. Experimental results will be compared to numerical simulations.

Perovskite ABO3 oxides display an amazing variety of phenomena that can be altered by subtle changes in the chemistry and internal structure. Most undergo non-polar structural distortions associated with a rotation of the BO6 octahedra about one or more of the crystal axes. These distortions are well known to control the charge/orbital, magnetic and electronic degrees of freedom. This strong coupling represents an opportunity to understand and create new functional materials that respond to an external perturbation in a useful way. For example, if octahedral rotations can be designed to induce ferroelectricity, an applied electric field would be able to directly couple to the BO6 octahedra, thereby controlling emergent phenomena such as magnetism, and possibly controlling metal/insulator transition. In this talk I will discuss our recent work in this area, highlighting the opportunities and the challenges to realizing such materials

Ubiquitous in a wide range of nature processes and technologies, a subtle modification (e. g. structure, element, chargehellip;) near an interface can have a decisive effect on emerging phenomena of the collective as well as each individual. Heterointerfaces of complex oxides exhibiting fascinating emergent phases and states due to numerous combinative contributions of atomic structures and chemistries manifest such interfacial subtlety, which can be effectively harnessed for the design of advanced materials with novel properties and accelerating materials integration into advanced devices. In this talk, I will demonstrate how to obtain atomic mapping of oxide heterointerfaces with sub-Ångstrom resolution by phase retrieving coherent Bragg rods, wherein complete atomically structural information of heterostructures hidden, utilizing synchrotron-based surface/interface X-ray scattering techniques. Firstly, I will give a brief introduction of surface/interface X-ray scattering techniques and phase-retrieval direct methods, in particular the COBRA method. In the following, I will demonstrate a few representative cases in the exploration frontier of complex oxide systems by applying the COBRA method, such as revealing structural motifs responsible for 2DEG and superconductivity adjacent with heterointerfaces, differentiating at the atomic-layer level the complicated elemental-specific cation distribution across buried interfaces, and depth-resolved mapping oxygen-octahedral tilt/rotation patterns essential with incipient functionalities of heterostructures. In the end, I will give a short commentary on likely future opportunities in X-ray studies of oxide interfaces and heterostructures enabled by the exciting advancement of new generation light sources.

Octahedral rotations in perovskite oxides (ABO3) are increasingly recognized as important contributors to the materials properties, along with effects such as epitaxial strain.[1] At interfaces between materials with different tilt systems, new distortions can arise that can modify properties at the interface or even the whole film [2], therefore it is crucial to study tilt behavior at interfaces on the atomic scale.
We have investigated octahedral tilt behavior in (LaCoO3)n/(SrTiO3)m superlattices (n = 5, and m = 5, 15, referred to as L5S5 and L5S15, respectively, below) and thin films on different substrates. The octahedral tilts were examined using annular bright field (ABF) images combined with annular dark field (ADF), allowing us to determine octahedral tilt with unit cell resolution.[3] We find that in the L5S5 superlattice, LCO layers induce octahedral tilts in the STO layers, which are not tilted in the bulk, while for the L5S15 superlattice it does not happen, suggesting a critical thickness for this interaction. We have further studied critical thickness effects using LCO films of different thicknesses. We found that in 15 u.c. LCO films on STO, the tilted structure is fully developed, and octahedral tilting even propagates into the last 2 unit cells of the STO substrate. These results were also supported by in-plane La-La lattice spacing modulations observed in HAADF images. However, 5 u.c. LCO films on STO were not tilted, suggesting a critical thickness in the same range for films as for superlattices. Interestingly, the critical thickness is substantially increased when the films are grown on LSAT substrate (a=3.87 Å), which induces less tensile strain compared to a STO substrate (3.905 Å): even for a 15 u.c. film, a fully tilted structure is observed.
These results will be discussed in comparison with first principle studies to show the interrelations between specimen thickness, epitaxial strain, and octahedral tilts in LCO, thus helping develop predictive models for this behavior at nanoscale.
* Research supported by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Division of Materials Sciences and Engineering, and through a user project supported by ORNL&’s Center for Nanophase Materials Sciences (CNMS), which is also sponsored by DOE-BES.
References
[1] Rondinelli, J.M. et al. Advanced Materials 24 1961 (2012).
[2] J. He et al., Phys. Rev. Let. 105 227203 (2010).
[3].Y.-M. Kim et al. Advan. Mater. 25 2497 (2013).

Structural distortions in the oxygen octahedral network in transition-metal oxides play crucial roles in yielding a broad spectrum of functional properties, and precise control of such distortions is a key for developing future oxide-based electronics. Here we show that interfacial octahedral connectivity is a determining parameter for these distortions and consequently for control of structural and electronic phases of a strained oxide film. Our complementary annular dark- and bright-field imaging in aberration-corrected scanning transmission electron microscopy[1] has successfully identified the slight octahedral distortions at the SrRuO₃/GdScO₃ heterointerface[2,3]. The detected picometer-order displacements of the oxygen at the interface, that is the interfacial octahedral connectivity, are closely tied to structural and electronic properties of the heterostructures. We further demonstrate the interface engineering of octahedral connectivity by inserting only a unit-cell-thick BaTiO₃ layer between SrRuO₃ and GdScO₃[4]. This provides a further degree of freedom for manipulating structural and electronic properties in strained oxide films, allowing for the design of novel oxide-based heterostructures.
[1] R. Aso, D. Kan, Y. Shimakawa, and H. Kurata, Scientific Reports. 3, 2214 (2013).
[2] D. Kan, R. Aso, H. Kurata, and Y. Shimakawa, Adv. Funct. Mater. 23,1129 (2013)
[3] D. Kan, R. Aso, H. Kurata, and Y. Shimakawa, J. Appl. Phys. 113, 173912 (2013)
[4] R. Aso, D. Kan, Y. Shimakawa, and H. Kurata, submitted (2013)

In complex oxides, the intriguing electronic, magnetic and orbital properties often result from how the oxygen anions surround the transition metal cation. Altering this bonding geometry, and thus the transition metal crystal field, can stabilize new and exciting ground states. Here, we present a novel method to tune the positions of the oxygen anions--and, thus, the crystal field--by creating polar interfaces within a single thin film material. By using the atomic monolayer control of molecular beam epitaxy (MBE), we are able to introduce “artificial” interfaces into a thin film of LaSrNiO4--a material in which the La and Sr dopant cations are usually randomly arranged over the A-sites. Using MBE, we interleave full layers of SrO (+0) and LaO(+1) in a series of chemically equivalent LaSrNiO4 films, varying the pattern of SrO and LaO layers relative to the NiO2 layers. This technique allows us, in one material, to capitalize on the polar interface phenomena found in more traditional multi-component systems (e.g. LAO/STO). Through synchrotron surface x-ray diffraction and Coherant Bragg Rod Analysis (COBRA) performed at the Advanced Photon Source, we directly investigate the La and Sr cation order and the resulting atomic displacements throughout the film thickness for each ordering pattern. We correlate these results with theoretical calculations and transport measurements of the layered nickelate films. For a particular interface pattern, we find that the nickel-oxygen bond lengths change by as much as 10% compared to the random alloy control films. The ability to modify the bond lengths by such a significant amount, while still maintaining the overall chemical equivalency of the material, could have broad implications for re-envisioning the electronic, magnetic and orbital properties of well-known oxide materials.

Recently there has been large focus on functional interfaces in perovskite heterostructures with tailored properties. In order to understand the properties of such interfaces it is important to probe both the crystal and electronic structure. Here we report on a combined scanning transmission electron microscopy - electron energy loss spectrum (STEM-EELS) study, relying on aberration-corrected TEM, and ab-initio study on the interface of (001) La0.7Sr0.3MnO3 / SrTiO3 perovskite heterostructures. The thin films are grown by pulsed laser deposition, and X-ray analysis confirms that they are epitaxial. By probing the electronic structure with energy-loss near edge spectroscopy (ELNES) we show that the valence of the Mn atoms is lower in the vicinity of the interface as compared to the bulk of the film. Taken together with the change in lattice parameters, as probed by high-resolution STEM and geometric phase analysis, and ELNES of the oxygen K-edge revealing an increased oxygen vacancy density at the interface, we show that the changes are consistent with an unordered Brownmillerite structure at the La0.7Sr0.3MnO3 side of the interface. We will corroborate the experimental data with VASP ab-initio studies of the interface. These findings give new insight into how oxygen vacancies can be responsible for an often-reported magnetic dead-layer observed in similar systems.

In many of publications, between 50 and 100 per year for the last five years, the Poole-Frenkel-effect (PFE) is identified or suggested as dominating current mechanism to explain measured current - electric field dependencies in metal-insulator-metal (MIM) thin film stacks. Very often the insulating thin film is a metal oxide as this class of materials has many important applications, especially in information technology. In the overwhelming majority of the papers the identification of the PFE as dominating current mechanism is made by the slope of the current - electric field curve in the so-called Poole-Frenkel plot, i.e. logarithm of current density, j, divided by the applied electric field, E, versus the square root of that field. This plot is suggested by the simplest current equation for the PFE, which comprises this proportionality (ln(j/E) vs. radic;E) leading to a straight line in this plot. Only one other parameter (except natural constants) may influence this slope: the optical dielectric constant of the insulating film.
In order to identify the importance of the PFE we have performed simulation studies of the current through MIM stacks with thin insulating films and have compared the current- electric field curves without and with implementation of the PFE. For the simulation an advanced current model has been used combining electronic carrier injection/ejection currents at the interfaces, described by thermionic Schottky emission, with the carrier transport in the dielectric, described by drift and diffusion in a wide band gap semiconductor. Besides the applied electric field (or voltage) many other important parameters have been varied: the density of the traps (with donor- and acceptor-like behavior); the zero field energy level of the traps within the energy gap, the energy level is changed by the PFE (also called internal Schottky effect); the thickness of the dielectric film; the permittivity of the dielectric film simulating different oxide materials; the barriers for electrons and holes at the interfaces simulating different electrode materials.
The main results and conclusions are: 1) NONE of the simulated current density curves shows the expected behaviour of the PFE. 2) In most cases within the tested parameter field the effect of PFE is negligibly small so that the current curves are identical with and without implementation of the PFE. 3) The only exception from this rule are the cases for which the trap energy level is close to the interface barrier energy determining the Fermi level in thin films close to the interfaces. 4) For the last cases some indication for a behavior towards that expected for the PFE is recognizable for large film thickness (>250 nm) combined with very high trap densities and low permittivity. The conclusion from all these results is: the observation of the PFE as dominating current mechanism in MIM stacks with thin dielectric (oxide) films (typically 30 nm) is highly improbable!

Epitaxial thin-film strain has enabled dramatic control over the structure and properties of ferroelectrics and studies of such materials have provided priceless insights into the physical response of these materials. Modern ferroelectric films, including bilayer and superlattice heterostructures, provide access to exotic structures and properties not available in the bulk. In this presentation, we will explore new modalities of strain control of thin-film materials that go beyond traditional lattice mismatch effects and how this can be used to enhance performance, independently tune susceptibilities, and provide new insights into the nature of these complex materials. Of particular interest, will be the discussion of the role of thin-film chemistry in controlling strain evolution, relaxation, and ultimately the properties of these materials. We will explore model systems including SrTiO₃ and BaTiO₃ where control of the laser fluence during the pulsed-laser deposition growth process can induce dramatic variations in the cation chemistry, induce deterministically controllable defect structures and densities, and can provide a pathway to extend thin-film strains to thickness orders-of-magnitude larger than expected. Within this context we will explore how one can couple epitaxial strain to defect structures to provide an additional out-of-plane strain component that can dramatically enhance ordering temperatures in ferroelectrics. At the same time, we will highlight the use of compositionally graded heterostructures to further extend what can be done with epitaxial strain. In particular we will highlight work on compositionally graded versions of PbZr_1-xTi_xO₃ and Ba_1-xSr_xTiO₃ where careful control of lattice mismatch and chemistry combine to produce strain gradients >10⁵ m^-1, exotic properties, and new approaches to independently control traditionally coupled properties. Building off of this work we will explore other artificial heterostructure routes by which to produce strain gradients in materials, the physical mechanisms for the observed effects (including delving into the role of the depolarization, gradient, and flexoelectric free-energy terms in driving property evolution), and how strain is accommodated throughout these films. Finally, we will explore the use of different film orientations and anisotropic in-plane lattice parameters in controlling domain structures and the relative contributions (be that intrinsic, extrinsic, secondary, and others) to the susceptibility and switching processes of ferroelectrics. Overall the presentation will highlight the role of thin-film epitaxy in driving the development of understanding of a complex set of materials with numerous technological applications.

The last decade has witnessed explosive growth of research on various oxide heterostructures, resulting in the discovery of exciting interfacial phenomena. However, it remains elusive hitherto to control and subsequently measure oxygen sub-lattice in oxide thin films/heterostructures that plays a pivotal role in the structure-property affairs. Specifically, the crystallographic mismatch, violation of local charge neutrality and the difference in chemical potentials between the constituents belonging to thin films/heterostrutures, strongly modify the atomic/electronic structure by - (i) electronic and/or atomic reconstruction (ii) formation of oxygen and/or cation vacancies as well as (iii) large atomic displacements. We present our recent results on ultrathin SrCuO2 cuprate layers that undergo profound change in the oxygen sub-lattice structure giving rise to novel structural and electronic properties.
The infinite-layer SrCuO2, in the bulk-form, has each Cu2+ ion coordinated by four planar O2- ions. However, for ultrathin layers, we find a transformation from bulk-planar to chain-type structure having apical oxygen via atomic re-arrangement [1,2]. This phenomena turns out to be a direct consequence of electrostatic instability associated with the polar nature of SrCuO2. Our detail investigation by X-ray diffraction, X-ray photoelectron diffraction, scanning transmission electron microscopy and X-ray absorption spectroscopy demonstrate that the oxygen sublattice can be built by design. We illustrate that the geometry of CuO4 plaquette in polar cuprates can precisely be controlled from in-plane to out-of-plane upon reducing the film thickness at the sub-unit cell level. This opens possibilities to design structures that have specific functions, e.g. current-carrying layers, charge reservoirs and scaffolding layers and realize novel electronic properties in the artificial oxide heterostructures.
References:
1. D. Samal, B. Kuiper, G. Rijnders, G. Koster, et al., Phys. Rev. Lett., 111, 096102 (2013)
2. B. Kuiper, D. Samal, G. Rijnders, G. Koster, et al., APL Mater. 1, 042113 (2013)

The temperature dependent current-voltage (I-V) characteristics of Ba0.8Sr0.2TiO3 (BST) thin-film capacitors with and without ZrO2 layer were studied in the temperature range of 400 to 450 K. Both BST film and BST/ZrO2/BST multilayered film were deposited on Pt/Ti/SiO2/Si substrates to make metal-insulator-metal (MIM) devices by a sol-gel process. It is observed that the leakage current is reduced for capacitors with ZrO2 layer compared to those without this layer. The leakage current is dominated by Ohmic conduction mechanism in the low electric field region for both films. The activation energy (Ea) for Ohmic conduction process is 0.5 eV for BST films whereas 0.56 eV for BST/ZrO2 multilayered film. The leakage current density is reduced by an order of magnitude for BST/ZrO2 multilayered films compared that of BST films in the whole electric field range. The estimated trap density (Nt) decreases from 5.1 x 1018 to 2.2 x 1018 cm-3 with the insertion of ZrO2 layer. The observed reduced leakage current for capacitor with BST/ZrO2 multilayer dielectric is due to the increase in the activation energy and decrease in the trap density and hence suitable for applications in dynamic random access memory devices.

We successfully fabricated the dual channel operation ambipolar transistor. Although ambipolar thin-film transistors (TFTs) with bilayer structure have been reported previously, to the best of our knowledge, the present work is the first to explore the hetero-layer source/drain (S/D) contact to the dual channel operation ambipolar transistor. Solution processed indium zinc tin oxide (IZTO) was firstly deposited on the 100-nm thick silicon dioxide (SiO2) as a n-type semiconductor and then dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) was thermally evaporated as a p-type semiconductor. For hetero-layered S/D contact, thermally evaporated Al was located between IZTO and DNTT. Molybdenum oxide (MoOx) covered on top of Al by tilting the shadow mask in two opposite direction to improve hole injection efficiency from Al to DNTT. To investigate the contact effect for both metal oxide and organic semiconductor layer, we performed the transmission line methods to extract contact resistance and confirmed hetero-layered S/D contact is effective for both p-type and n-type operation. By introducing hetero-layered S/D contact, charges can be directly injected from S/D electrodes to both metal oxide semiconductor layer and organic semiconductor layer without tunneling or percolation effects in semiconductor layer. Hetero-layered S/D contact also effectively canceled out the capacitance difference in metal oxide and organic semiconductor layer by shortening the channel length to organic layer. As a result we successfully balanced the p-type operation and n-type operation on currents (1.05 µA in p-type operation and 0.16 µA in n-type operation).

ZnO-based oxide semiconductor thin film transistors (TFTs) have attracted a considerable attention as the backplane electronics for active matrix organic light emitting diodes (AMOLEDs). Despite of these advantages, the detrimental problem of oxide TFTs is a threshold voltage instability under gate bias stress condition. The degradation of electrical characteristics under gate bias stress has been explained by the carrier trapping, injection, ambient interaction and oxygen vacancy model [1]. This study examined the effect of high pressure annealing (HPA) treatment on bottom gate indium gallium zinc oxide (IGZO) TFTs with a SiO2 etch stopper layer because of oxygen vacancy concentration can be reduced by HPA in oxidizing atmosphere.
The O2 gas and H2O vapor has been used as a oxidant species during HPA threatment to improve the photo bias stability of metal oxide TFTs [2,3]. In this study, the hydrogen peroxide (H2O2) was introduced to the IGZO channel during HPA process because the oxidation power of H2O2 is expected to be larger than that of O2 or H2O. The H2O2 treated device exhibited the superior stability to that of H2O treated device : the Vth shift was reduced from 2.3V (H2O HPA) to 1.2V (H2O2 HPA) under positive bias stress condition. This improvement can be attributed to the reduce concentration of oxygen vacancy defects in the IGZO channel layer during the H2O2 HPA treatment.
[1] J. K. Jeong, J. Mater. Res., 28, 2071 (2013).
[2] S. Y. Park, J. H. Song, C. -K. Lee, B. G. Son, C. K. Lee, H. J. Kim, R. Choi, Y. J. Choi, U. K. Kim, C. S. Hwang, H. J. Kim, and J. K. Jeong, IEEE Electorn Device Lett., 34, 894 (2013).
[3] K. Nomura, T. Kamiya, M. Hirano, H. Hosono, Appl. Phys. Lett., 95, 013502 (2009).

CdO is an indirect gap semiconductor with an extremely low conduction band edge (~5.8 eV w.r.t vacuum level E_Vac). Nominally undoped CdO films are highly n-type with carrier concentrations of 2×10¹⁹ - 3×10²⁰cm^-3. On the other hand, SnTe is known to be a p-type semiconductor with a narrow band gap and high valence band edge at ~4.4 eV w.r.t E_Vac. Hence CdO/SnTe heterostructure have an extreme type III band offset where the valence band edge of SnTe is more than 1 eV higher than the conduction band edge of CdO. Therefore, at the interface of a CdO/SnTe heterostructure, it is expected that electrons can transfer from the valence band of SnTe to the conduction band of CdO. The electron transfer effect may allow us to achieve a material with higher or tunable carrier concentration without the formation of compensating native defects.
In this work, we carried out an investigation on the interface properties of the CdO/SnTe heterostructures. A series of CdO/SnTe multilayer structures with various SnTe and CdO thicknesses were deposited by sputter deposition. Electrical and optical properties of these structures were studied by Hall effect measurements, photoluminescence and optical reflectance. We have found that the electrical properties of CdO/SnTe films depend not only on the relative thickness of the films but also on the sequence of the layer deposition. The apparent carrier type and carrier concentration in CdO/SnTe films can be changed by changing of the thickness of both SnTe and CdO layers. Charge transfer properties at the interface of the CdO/SnTe heterostructure can be deduced from the thickness dependence of electrical properties in CdO/SnTe films. We analyze Hall effect results of the CdO/SnTe heterostructures by a multilayer model and discuss the charge transfer properties at the interface of CdO/SnTe films in terms of the band offset of the materials and the amphoteric native defect model.

We were able to systematically control crystallographic characteristics and electrical and magnetic properties of nickel oxide epitaxial thin films integrated with cubic yttria-stabilized zirconia (c-YSZ) buffered silicon(001) substrates. The NiO epilayers were grown under several oxygen partial pressures by pulsed laser deposition. The out-of-plane orientation of the NiO layers showed an interesting behavior where it changed from <111> at lower pressures (7×10-6 Torr) to <100> at higher pressures (5×10-2 Torr). This observation was attributed to the nature of surface termination and templating effect of the c-YSZ{100} platform at different pressures. Using theta;-2theta; and phi; scans of X-ray diffraction, the epitaxial alignment across the NiO/c-YSZ interface was determined to be {111}NiO||{100}c-YSZ and <110>NiO||<100>c-YSZ for the heterostructure grown under a low pressure and {100}NiO||{100}c-YSZ and <100>NiO||<100>c-YSZ for the heterostructure grown under a high oxygen pressure. Our high resolution TEM studies revealed formation of atomically sharp interfaces with no evident of interfacial reaction and confirmed the established epitaxial relationships. An n-type electrical conductivity was observed in the NiO epilayers grown under lower pressures which was turned p-type in the films deposited under higher oxygen partial pressures. Besides, the electrical resistivity increased with the growth pressure. These observations were correlated to the nature of point defects in the NiO crystal. The formation of oxygen vacancies and metallic nickel at lower pressures and excess oxygen and trivalent nickel at higher pressures was revealed by XPS. We established a structure-property correlation in NiO/c-YSZ{100}/Si{100} thin film epitaxial heterostructures with especial emphasis on the stoichiometry and crystallographic characteristics.

Persistent photoconductivity after UV exposure is a well-known effect in crystalline ZnO, but not commonly characterized in amorphous thin films. This is attributed to photoinduced holes reacting with and desorbing the electron-trapping oxygen ions on the ZnO surface, as well as inducing photolysis of ZnO and generating free electrons. Persistent photoconductivity has been observed to be stronger in a-ZnO than in crystalline ZnO, and we find using impedance measurements that in a-ZnO, UV exposure creates mobile positive ions that contribute to this conductivity. We take advantage of this behavior to create UV photodiodes with conductivity that stays high for some time after UV exposure allowing for a longer polling interval and lower power use. The conductivity in these devices can be more rapidly reset by exposing them to oxygen. Finally, the behavior of a-ZnO TFTs suggests that an oxygen blocking layer could be used to alter the conductivity recovery rate under ambient conditions.

Thin films of (Pb_1-xLa_x)(Zr_0.52Ti_0.48)O₃ (PLZT) of high crystalline quality and La doping levels of (atomic %) 0.1% , 0.5% and 1% were grown on SrTiO₃ (100) (STO) substrates and the ferroelectric properties were compared with thin films of PbZr_0.52Ti_0.48O₃ (PZT) grown on same type of substrate. PZT is a leading material in industry, used for micromechanical actuators, electro optics, sensors and many more applications. It is also one of the best candidates for nonvolatile memory devices due to its very good ferroelectric (FE) behavior, especially at the morphotropic phase boundary(MPB).
In order to characterize the ferroelectric behavior, thin film capacitors were grown with top and bottom electrodes of La_0.7Sr_0.3MnO₃ (LSMO). The pulsed laser deposition system (PLD) used to grow the films utilizes a UV laser of wavelength 248 nm with a pulse frequency of 10 Hz. The fluence at the target for LSMO and PLZT was 2 J/cm² and 3 J/cm² respectively. The process was carried out under O₂ pressure with substrate temperatures of 550°C and 720°C for PLZT and LSMO respectively.
X-ray diffraction (XRD) data revealed epitaxial film growth and atomic force microscopy images showed smooth surfaces with roughness (rms) values in the nanometer range. In-plane compressive strains, measured using XRD strain analysis, were observed to be higher, at lower La concentration levels which results in enhanced tetragonality in the PLZT unit cell compared to PZT. High resolution TEM images of the PLZT/LSMO interface revealed atomically sharp and flat interfaces. The polarization vs. voltage hysteresis loop showed enhanced remnant polarization (P_r) and lower coercive fields (E_c) for all levels of La doping compared to undoped PZT films. The highest enhancement in P_r was obtained for the 0.1% La doped sample at 91 µC/cm² at E_c of 32 kV/cm.
The increased tetragonality due to compressive strains and defect dipoles created from the residual ordering of charges at the La³⁺ sites were suspected to be contributing factors in the observed enhancement in polarization.

Sensors research has being intensified over the past decades, due to their extensive use in several fields, including medical applications [1]. Biosensors are useful to identify and quantify small amounts of specific species. Actually, oxide thin films have been widely studied to be used as sensitivity layers of biosensors. In this study we used the fluorine-doped tin oxide (FTO) thin films as the sensitive material of an extended gate field effect transistor (EGFET) device, to be applied as a ionic sensor. The mains focus was its use as pH sensor. The electrical characterization of sensors using different effective areas was performed. Comparisons with other materials and results from the literature were performed in order to look for a final standardization. That will help the understanding of the relevant issues in the miniaturization of these kinds of devices to be used in medical applications. Two different experiments were performed: i) the first one used four different films with areas ranging from 42 to 230 mm2 totally immersed in the buffer solution (called TA - Total Area), and ii) the second one used an independent film with 230 mm2 area partially immersed in the buffer solution with different exposed areas. These areas ranged from 60 to 230 mm2 (called PA - Partial Area). This second type of measurement had an additional variation where the sample was either inserted or removed from the solution. The results will be discussed based on the interpretation of the variations of the electric potential difference (ΔV) between the bulk of the solution and the surface of the sensing film. ΔV is the addition of several components, such as the potential created between the Gouy-Chapman region and the bulk, the potential created between the Helmholtz plane in the surface/electrolyte interface [2] and the potential inside the FTO film. According to the results, larger areas lead to the smaller ΔVs. The sensitivities of the devices also presented significant variations as a function of electrode area. The larger the area, the higher the sensitivity. For a fixed area though, higher sensitivities were obtained for PA measurement configuration. All effects will be discussed including a broad comparison with data extracted from recent literature.
[1] J-J. Xu, X-L. Luo, and H-Y. Chen, “Analytical aspects of FET-based biosensors,” Front. in Biosc., vol. 10, pp. 420-430, Jan. 2005.
[2] R. Morrow, and D. R. McKenzie, “The time-dependent development of electric double-layers in pure water at metal electrodes: the effect of an applied voltage on the local pH,” Proc. of Royal Soc., vol. 468, Mar. 2013.

Zinc Oxide (ZnO) is a versatile wide band gap (Eg = 3.3 eV) II-VI semiconductor material that finds applications from gas sensors to optoelectronics and spintronics. In recent years, it has attracted considerable attention as a promising candidate for next generation nonvolatile memory applications due its resistance switching property. Though there are many reports of resistance switching in ZnO based devices, in most of cases, textured or polycrystalline ZnO films have been used and there is still no clear understanding of the switching mechanism. In order for these devices to be useful in real applications, a clear understanding of the origin and mechanism of resistance switching is needed. We have prepared RRAM devices by PLD with layers consisting of Pt/ZnO/TiN on Si(001) substrates. The microstructure of these multilayers and devices was characterized by XRD (theta;-2theta;, phi; scans) and HRTEM. The ZnO and Pt films were grown bi-epitaxially on an epitaxial TiN layer. The epitaxial relation between various layers in the device is given as (111)Pt #8214; (0001)ZnO #8214; (001)TiN #8214; (001)Si and [100]TiN #8214; [100]Si, [010]ZnO1 #8214; [110]TiN , [100]ZnO1 #8214; [1-10]TiN and [ 10-1]Pt1 #8214; [100]ZnO1. Electrical measurements were performed at room temperature using an HP 4155B semiconductor parameter analyzer fitted with a micromanipulator probe station. The devices showed reproducible resistance switching that was critically dependent on the ZnO deposition conditions. A thin layer of oxide of zinc and titanium at the interface between TiN/ZnO was found to control the switching of resistance in these devices, thus confirming the switching to be an interface-dependent phenomenon and not due to filamentary conduction pathways in the structures. The correlated structural and electrical properties of epitaxial Pt/ZnO/TiN/Si devices are discussed.

Integration of perovskite oxides onto silicon and III-V substrates has been a much sought after topic to realize novel electronic devices with increased functionalities. Research on the growth and properties of oxide thin films has been done using SrTiO3 (STO) and other relevant oxide substrates with relatively little work on their integration with semiconductors that includes silicon and III-V substrates. Such integration would combine high speed optoelectronics and signal processing with the multifunctional properties of oxides. Molecular Beam Epitaxy with solid sources was used to grow crystalline oxides on Si, GaAs and InP utilizing 2 chambers; one As-based III-V and one oxide connected using a UHV buffer line. Molecular oxygen and co-deposition was used for all the oxide growths. In addition to GaAs and Si the growth of STO on InGaAs lattice matched to InP is demonstrated. Due to their high frequency and low noise properties, InP based HEMT's have already been demonstrated for efficient use in a variety of devices (amplifiers, low power applications, radars etc). In combination with these properties, InP is a direct band gap semiconductor and posses a relatively high electron mobility making for a perfect substrate to realize novel electronic devices by the inclusion of added functionality brought forth by STO and other perovskite oxides.STO by itself has ferroelectricity under strain with large lattice mismatch, the critical thickness can be very small for device fabrication. However using thin STO on III-V as virtual substrates, further growth of perovskite oxides with multi-functionalities can be undertaken.
Using this virtual substrate approach, BiMnO3 was successfully grown onto silicon and BaTiO3 on GaAs. Manganese based perovskites are famously known for forming a major portion of the class of materials called multiferroics. These have the coexistence of ferroelectricity and ferromagnetism. Low powered memory devices, tunnel junction transistors, high density memory and ultra sensitive sensors are some of the few applications that we can attribute to multiferroics. The successful integration of these onto current silicon technology will accelerate realization of these properties with ease. RHEED is used to investigate the surface reconstructions, while XRD and TEM will be used to characterize the structural properties. X-Ray Photoelectron Spectroscopy (XPS) is to investigate both the elemental composition, including interface chemistry and cation oxidations. Results of electrical and magnetic characterizations of the oxides that is integrated with semiconductors will be presented.

Abstract
The ZnO/Zn0.85Mg0.15O asymmetric double quantum well (ADQW) and multiple quantum well (MQW) were fabricated with the method of plasma assisted molecular epitaxy on c-plane sapphire [1,2]. The widths of the narrow well (NW) and the wide well (WW) of the ADQW are chosen to fascinate rapid LO phonon-assisted carrier tunneling from NW to WW. The room-temperature optical properties and optical pumped lasing have been studied. As compared to the MQW sample, the lasing threshold of the ADQW sample is reduced by a factor of ~ 3 times to 6.0 kW/cm2. Optical gains measured through the variable striple-length measurement reveals that the gain of the ADQW is enhanced as compared to that of the MQW. The low-threshold of the ADQW sample is proposed to be due to the exciton density enhancement at the WW. This result suggests that ZnMgO ADQWs is a promising structure for realizing of ultralow-threshold exciton-based laser device.
Reference
[1] S. C. Su, Y. M. Lu, G. Z. Xing, and T. Wu, Superlattices Microstructure 48, 485-490 (2010).
[2] S. C. Su, H. Zhu, L. X. Zhang, M. He, L. Z. Zhao, S. F. Yu, J. N. Wang, and F. C. C. Ling, Appl. Phys. Lett. 103, 131104 (2013).

Ferroelectrics have been applied to various applications such as energy harvesting system, information storage and actuator, etc. Although there has been significant amount of research for these applications, most of them have been performed using rigid substrates. However, as increasing demand for flexible devices, much interest has been aroused in the fabrication of the ferroelectric materials on the flexible substrates. Nonetheless, a typical preparation method for the flexible structures needs very complicated manufacturing process. In this presentation, we demonstrate flexible ferroelectric Pb(Zr,Ti)O3 (PZT) thin structures with largely enhanced ferroelectric/piezoelectric properties using a simple preparation method with a metal paper substrate. The PZT thin films were prepared by sol-gel method on the flexible metal paper substrates. Piezoresponse force microscopy (PFM) and P-E hysteresis loop measurements were performed to examine piezoelectric and ferroelectric properties of the prepared ferroelectric films. The PZT thin films on the metal paper show higher piezoresponse and remanent polarization compared to those on the conventional Platinized substrates. Furthermore, the hysteresis loops of the PZT thin films on the metal paper show more rectangular shape. We also show good performance of a flexible energy harvesting device prepared on the metal paper. The obtained results are expected to provide a simple fabrication method to realize flexible ferroelectric devices for further applications.

In-doped ZnO (IZO) films were prepared by atomic layer deposition (ALD), and the ALD-grown IZO films with (or without) N2 annealing were employed to serve as transparent conducting layers (TCLs) in InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs). Based on theta;-to-2theta; X-ray diffraction (XRD) analyses, the lattice constant c of a N2-annealed IZO was detected to be larger than that of the ZnO. Hall data show that as-deposited and N2-annealed IZO films are all heavily n-type doped with room temperature (RT) electron concentration in the range of mid 1020 cm-3. It was also found that N2 annealing tended to enhance the electron mobility of the IZO film so that a minimized film resistivity at mid 10-4 Omega; cm was reached.
It appears that all the forward-biased InGaN/GaN MQW LEDs with ALD-grown IZO TCLs exhibit good current spreading property. In particular, the implementation of N2-annealed ALD-grown IZO in an InGaN/GaN MQW LED results in light extraction enhancement and forward voltage reduction of the LED under certain conditions. At 20 mA operating condition, the 600 C N2-annealed n-IZO-coated InGaN/GaN MQW LED was found to exhibit an optimized forward voltage of 3.2 V with the specific contact resistance of n-IZO/p-GaN contacts being 8.8×10-3 Omega;-cm2. The 600 C N2-annealed n-IZO-coated InGaN/GaN MQW LED was also found to show a smaller ideality factor value than that of an InGaN/GaN MQW LED structure having a commercial-grade indium tin oxide (ITO) TCL. Comparing with the ITO-coated InGaN/GaN MQW LED, the 600 C N2-annealed n-IZO-coated InGaN/GaN MQW LED was found to show a slightly lower light output power owing to its lower optical transmittance at 450 nm which neutralizes its light extraction advantange enhanced by the larger refractive index value of IZO.

We have fabricated top-gated metal oxide thin-film transistors (oxide-TFTs) with reverse-offset-printed (ROP) silver (Ag) source/drain(S/D) electrodes. Ag metal electrode having 20~50micrometer channel length was printed by reverse offset printing (ROP) using nano-silver paste ink for the S/D of Indium zinc oxide transistors (IZO-TFT). The Ag electrode was applied to fabricate top-gate/bottom contact IZO devices, which showed reproducible Oxide-TFT characteristics such as the field-effect mobility of sim;10minus;2 cm2/Vs at maximum process temperatures as low as 250oC. Specific resistance and surface roughness of printed Ag electrodes with increasing curing temperature were investigated, and surface morphology and grain growth mechanism were systematically verified using a scanning electron microscope (SEM) and atomic force microscope (AFM), and an optical microscope in order to obtain an optimized ROP Ag electrode.

In general, seebeck coefficient(thermo-power) is a measure of the energy dependence of the DOS(density of state) at the fermi level(E_F). The DOS(at E_F) of transition metal oxides is predominated by heavy d-electrons, therefore, the conduction electrons are expected to be localized strongly. Correlated transition metal oxide material, strontium ruthenate(SrRuO₃) is known to be metallic itinerant ferromagnet and is also known for its widespread utility as a conducting electrode in oxide heterostructures. However, its thermoelectric properties have scarcely been reported until now. Here, we demonstrated that the SrRuO₃ thin film is responsible for photo-thermal electric current properties by scanning photocurrent microscopy. We estimated the seebeck coefficient of SrRuO₃ with measured photocurrent and also provide the temperature and optical power dependence. This study suggest that scanning photocurrent microscopy can be a useful tool for understanding thermoelectric properties of materials and also suggest the transition metal oxides including strontium ruthenate as a potential thermoelectric material.

Relaxor-ferroelectric 0.4Bi(Ni1/2Zr1/2)O3-0.6PbTiO3 (BNZ-PT) thin films were prepared on Pt(111)/Ti/SiO2/Si substrates via an chemical solution deposition (CSD) technique. The thin films exhibited good crystalline quality and dense, uniform microstructures with an average grain size of 50 nm. The dielectric, piezoelectric and ferroelectric properties of the films was investigated. The local effective piezoelectric coefficient d33 was 78.6 pm/V. Moreover, a giant energy-storage density of ~31.8 J/cm3 under an electric field of 2000 kV/cm was achieved at room temperature. Furthermore, the temperature dependence of the energy-storage density and effective d33 indicated that the film exhibited good temperature stability. The large piezoelectric response and high energy-storage density with a good stability make BNZ-PT films promising candidates for the energy harvesting and storage device applications.

We present some recent results on the steady state and transient electron transport that occurs within bulk alloys of zinc-magnesium-oxide. These results are obtained using an ensemble semi-classical three-valley Monte Carlo simulation approach. We find, for electric field strengths in excess of 180 kV/cm, that the steady-state electron drift velocity associated with these alloys exceeds that associated with bulk wurtzite gallium nitride. We also present evidence that suggests that the negative differential mobility exhibited by the velocity-field characteristic associated with alloys of zinc-magnesium-oxide is not related to transitions to the upper valleys. The transient electron transport that occurs within this alloy is studied by examining how electrons, initially in thermal equilibrium, respond to the sudden application of a constant electric field. From these transient electron transport results, we conclude that for devices with dimensions smaller than 0.1 microns, gallium nitride based devices will offer the advantage, owing to their superior transient electron transport, while for devices with dimensions greater than 0.1 microns, electron devices based on alloys of zinc-magnesium-oxide will offer the advantage, owing to their superior high-field steady-state electron transport. The device implications of these results will be explored. Our results show that the Monte Carlo simulations of the materials response to the instant change of the electric field could be used for establishing the figures of merit for materials applications for short channel ultra high-speed semiconductor devices.

A high electron concentration of a two-dimensional electron gas at a hetero-interface screens ionized impurities, enhances the low field mobility, and affects the electron drift velocity at high electric fields and during transients. In this paper, we examine what impact the presence of a two-dimensional electron gas has upon the electron transport within a zinc-oxide based hetero-junction. At first, we consider the band alignment of zinc-oxide with a number of different wide energy gap semiconductors. A review of some recent progress that has been made in understanding the nature of the electron transport that occurs within a zinc-oxide based two-dimensional electron gas is then provided. Monte Carlo simulations of the electron transport that occurs within this electron gas are employed for the purposes of this analysis; our results are obtained using an ensemble semi-classical three-valley Monte Carlo simulation approach. The free electron concentration within the two-dimensional electron gas is evaluated through consideration of both the spontaneous and piezoelectric components to the polarization. Steady-state and transient components of the electron transport will be considered for the purposes of this analysis. A comparison with results corresponding to the case of bulk wurtzite zinc-oxide will be provided. Finally, the device implications of these results will be considered.

We have used laser MBE to grow LaNiO3 atomic layer-by-layer from La2O3 and NiO targets on LAO and STO substrates. The reflection high energy electron diffraction (RHEED) spot intensity was used to control the growth of alternating atomic layers in order to achieve stoichiometric LaNiO3 thin films. X-ray diffraction, x-ray reflection, and atomic force microscopy were used to characterize the structure, thickness, and surface morphology of the films. The thickness dependent metal insulator transition was investigated with a combination of transport measurements and x-ray absorption spectroscopy. The latest results on the fabrication of complex nickelate heterostructures with this approach will be presented.

Owing to its narrow band gap (< 2 eV) and p-type conductivity delafossite CuFeO2 is attractive for applications in the field of solar energy conversion. In addition, low temperature (< 11 K) non-collinear incommensurate antiferromagnetic structure has been shown to be at the origin of a magnetoelectric effect. Obtaining pure phase CuFeO2 thin films, however, is relatively difficult because it is necessary to maintain the lowest possible Cu valency (+1) in order to avoid forming the comparably stable spinel compound CuFe2O4.
We present a systematic study of the pulsed laser deposition (PLD) growth conditions for epitaxial (001) oriented CuFeO2 thin films (rhombohedral, R-3m) on Al2O3 (00.1) substrates1. Formation of a secondary impurity phase composed of spinel CuFe2O4 occurred for high oxygen pressures or high growth temperatures. This impurity phase was removed completely by optimizing the growth conditions. Reflection High-Energy Electron Diffraction (RHEED), x-ray diffraction (XRD) and transmission electron microscopy (TEM) showed that the pure phase delafossite films are highly epitaxial to the substrate. The chemical purity was verified by Raman and x-ray photoelectron spectroscopies (XPS). The indirect bandgap of 1.15 eV was measured using infrared reflectivity, and is in agreement with the CuFeO2 bulk. Finally, we discuss the growth and structural characterization of delafossite multilayers, CuFeO2/CuGaO2.
This work was supported by a Research Challenge Grant from the West Virginia Higher Education Policy Commission (HEPC.dsr.12.29) and the Microelectronics Advanced Research Corporation (Contract # 2013-MA-2382) at WVU.
1T. Joshi, T. R. Senty, S. Chen, P. Ferrari, P. Borisov, X. Song, A. D. Bristow, A. L. Cabrera, and D. Lederman, J. Mater. Res., submitted (2013).

While molecular beam epitaxy and pulsed laser deposition are both powerful tools for materials synthesis, the relative simplicity of magnetron sputter deposition offers many important advantages, particularly for the growth of complex oxide materials. However, the presence of the plasma and high magnetic fields prevent use of the usual monitors employed for growth optimization and precise thickness control, such as reflected high energy electron diffraction. Furthermore, the atomic-scale processes governing the surface roughness and composition in the doped oxides remain poorly understood. Here, we describe results from recent in-situ synchrotron experiments on the growth behavior and structure of epitaxial La1-xSrxGaO3 thin films grown by off-axis rf magnetron sputtering. We demonstrate the utility of in-situ x-ray methods for determining optimal growth windows and describe the impact of Sr doping concentration on the growth mode. The effect of surface coverage on the kinetic processes occuring on the surface post-growth will be discussed.

Transparent conductive oxide (TCO) thin films are a very important component in solar cells, flat displays, light-emitting diodes and other optoelectronic devices. The primary requirement for TCO material is that it should have low electrical resistivity as well as high optical transparency. In this paper, Indium molybdenum oxide (IMO) thin film was prepared by pulsed DC magnetron sputtering on soda lime glass substrates with different substrate temperature. The structure properties were characterized by XRD, AFM and XPS. The electrical parameters such as resistivity, carrier concentration and mobility of IMO thin films were measured with Van der Pauw configuration. The optical properties were measured using a spectrophotometer. The temperature-dependent Hall measurement was also performed. The results demonstrated that all the IMO thin films exhibited the In2O3 bixbyite cubic structure. With the increase of substrate temperature, the resistivity increased initially and decreased at higher temperatures. The mobility showed an increasing trend in general and the maximum mobility of 53.9 cm^2V^-1s^-1 at 250 degree Celsius was obtained. The films prepared at higher substrate temperature showed good optical transmission in the visible range.

Tin-doped indium oxide (ITO) and Al-doped zinc oxide (ZnO:Al) are used as transparent electrodes for various flat panel displays such as liquid crystal displays, plasma displays, organic light emitting diodes, and thin film solar cells. In such applications, it is important not only to obtain specific electrical or optical properties, but also to precisely control the surface morphology and the microstructure to tailor the film performance. The microstructure and morphology evolution of ITO and ZnO:Al thin films deposited by radio-frequency magnetron sputtering from an oxidic target in different gas ambient (Ar, Ar/O2 or Ar/H2 mixture) were investigated by X-ray diffraction (XRD), X-ray reflectivity (XRR) and atomic force microscopy (AFM).
From the XRD results, we observed different texture growth in terms of excess or absence of oxidation of body centered cubic structure ITO films. The preferred orientation of ITO films prepared in pure Ar sputtering gas shows (111) plane while changes to (211) plane in thicker film. ITO films deposited in Ar/10% O2 ambient show strong (111) orientation, while crystal growth in the (100) plane is observed for Ar/10% H2 deposition. The film density obtained from XRR increases with O2 introduction while decreases with H2 introduction. Height-height correlation functions, derived from AFM profiles, were used to compare the microstructure scaling behavior for different ITO films. The interface width (w) grows with film thickness (dt), and exhibits a power law behavior, i.e. Rrms ~ dtβ. The roughness decreases with increasing O2 flow, while increases with increasing H2 flow. The growth exponent β is found to be 0.48, 0.75 and 0.95 for Ar/10% O2, pure Ar and Ar/10% H2 deposition, respectively. The correlation length increasing with film thickness also shows a power law of z = 0.36, 0.44 and 0.57 for these three different gas ambient separately. The roughness exponent α is found around 0.6- 0.8 for all deposition. This behavior is attributed to a reduced (or increased) surface mobility of the In adatoms under oxygen excess (or absence) growth conditions. For hexagonal structure ZnO:Al films, the roughness evolution shows contrary trend to ITO films. The growth exponent β decreases with H2 evolution which is found to be 1.47, 1.28 and 0.7 when deposited in pure Ar, Ar/5% H2 and Ar/10% H2, respectively. A combination of local and non-local growth models in 2+1 dimensions is discussed in ITO and ZnO:Al film growth in this work.

Atomic layer deposition (ALD) offers several advantages over other traditional methods of growth: it can be used at relatively low temperatures, and deposition of the layer is self-limiting, so the growth of only one layer at a time is guaranteed [M. Leskelä and M. Ritala, Thin Solid Films 409, 138 (2002); S. M. George, Chem. Rev. 110, 111 (2010)]. The ALD growth of SrTiO3 (STO) consists of dosing the hydrogenated TiO2 surface with the Sr precursor (Strontium, bis[1,2,4-tris(1-methylethyl)-2,4-cyclopentadien-1-yl], also known commercially as Hyper Sr). Then, the surface is dosed with water, which washes away the remainder of the precursor and leaves behind a hydrogenated SrO surface [M. D. McDaniel, A. Posadas, T. Q. Ngo, A. Dhamdhere, D. J. Smith, A. A. Demkov, and J. G. Ekerdt, J. Vac. Sci. Technol. A 31, 01A136 (2013)]. However, the exact atomic-scale mechanism of the adsorption of the precursor, and the how the water dosing removes the remainder of the precursor, is not well understood. Using density functional theory, we examine the structure of the Sr precursor, calculate its density of states, and determine its vibrational modes and frequencies. To model the adsorption of the precursor on the TiOnot;2-terminated STO surface, we use a 3 layer thick slab of STO, symmetrically (2radic;2 x 2radic;2) TiO2-terminated with a vacuum thickness of ~30 Å. We partially hydrogenate the top surface in order to simulate the chemical environment of the ALD growth process. We fix the bottom layer of TiO2; the other two layers are allowed to relax. We place the Sr precursor on the surface of STO, and examine the behavior of the adsorption of the Sr precursor. We also calculate the density of states of the adsorbed system, and comment on the likeliest route of adsorption.

Wide bandgap heterojunctions that can be deposited on crystalline silicon at low-temperatures are technologically interesting, e.g. the a-Si/c-Si heterojunction used in HIT solar cells. We recently reported such a heterojunction by depositing TiO₂/Si on Si(100) at only 100°C using CVD [S. Avasthi, et al., APL 102, 203901 (2013)]. Photoelectron spectroscopy shows that at the TiO₂/Si interface there is a large valence-band barrier of 3.4 eV but a small conduction-band barrier of 0.1 eV , which blocks holes in silicon but allows transport of electrons from Si to TiO₂ [J. Jhaveri, et al., 39th IEEE PVSC (2013)]. Here we (a) investigate the mechanism of TiO₂ growth on Si and (b) demonstrate that the TiO₂/Si interface is an almost perfect hole-blocker.
Mechanism: The TiO₂ deposition cycles consists of a cooling step to adsorb the Ti-precursor onto the Si surface, followed by a 100 °C step to thermolyze the precursor into TiO₂. Physical & electrical properties of the resulting TiO₂ layer strongly depends on the time and temperature of the adsorption step. The adsorption of Ti-precursor on Si is not self-limiting, so longer adsorption steps lead to thicker TiO₂ layers - increasing adsorption time from 1 to 30 min increases TiO₂ thickness from 1.5 to 3.5 nm. Thicker TiO₂ layers are more effective at hole-blocking, as evident from the IV characteristics of Al/ TiO₂/pSi/Ag diodes which show that J₀ reduces from 10^-6 to 10^-10 A/cm². The TiO₂ growth rate can be increased 2x by introduction of water, even in trace amounts. However, the blocking characteristics of the resulting TiO₂/Si are poorer, suggesting that the water-catalyzed heterointerface is more defective. The adsorption temperature has almost no effect on the TiO₂ layer thickness - for adsorption temperature from 5C to -10C (time = 10min), the deposited TiO₂ layer remains the same. However, the quality of the TiO₂ is better at lower adsorption temperatures - as evident from the IV characteristics of Al/ TiO₂/pSi/Ag diodes which show 4X reduction in J₀ as adsorption temperature is reduced from 5C to -10C. We hypothesize that the lower temperatures lead to more uniform precursor adsorption, due to better precursor stiction, resulting in fewer local thin or thick regions. The more uniform TiO₂ film is better at blocking holes, resulting in a lower J₀.
Hole-blocking efficacy: Using diode reverse-recovery method on the Al/TiO₂/pSi/Ag diode, we extract the recombination lifetime of electrons in silicon, and then calculate the expected current due to diffusion of electrons from the Al into the conduction band of Si. The calculated electron closely matches the total current, showing that almost all of the current in the Al/TiO₂/pSi/Ag diode is due to electron diffusion and the hole-current is almost zero, i.e. TiO₂/Si interface is a near-perfect hole-blocker.

Phase pure Cu₂O is desirable as an absorber material for solar cells using ZnO electrodes because of its reported bandgap (~2.1 eV), a suitable band alignment with ZnO, and the ability to dope both n- and p-type. Pulsed laser deposition (PLD) has been used to grow Cu₂O on ZnO and other substrates at low substrate temperatures and in oxygen ambient. Transmission electron microscopy and X-ray diffraction analyses showed a single phase Cu₂O with (111) and (200) textures while growing at 200 C and 25 C substrate temperatures respectively. Electrical measurements, including conductivity and electrochemical Mott-Schottky measurements, showed n-type behaviour with resistivity as low as ~3×10^-2 #8486;.cm, much lower than previously reported results at ~25 C without any oxygen injection in the PLD chamber. The as grown Cu₂O thin films also showed a transition from highly conductive n-type to highly resistive (~4×10⁴ #8486;.cm) p-type behaviour as the oxygen concentration increased. UV-Vis diffuse reflectance spectroscopy was used to estimate the optical bandgap of the polycrystalline thin films to be in the range 2.01 -2.10 eV.
The poster will describe the growth, structural and electrical properties of the Cu₂O films, and the properties of Cu₂O solar cells grown on ZnO electrodes.

Raman spectroscopy is a symmetry sensitive technique making it not only possible to study vibrational properties but also to determine crystal structures and orientations. Discrete variations of the angle between the incident and scattered polarization vectors with respect to the crystal lattice of the probe under study in the Raman experiment allow one to gain further insight into material properties. By analyzing the full angle and polarization dependence by this novel approach one can yield much more insight than just a proof of selection rules or identification of lattice modes. This will be shown exemplary for three different oxide semiconductor systems grown either by molecular beam epitaxy (MBE), chemical vapor deposition (CVD), or sputtering: ZnO:N, Cu₂O, and SnO.
We show that rotational Raman spectroscopy (RoRa) proofs the formation of Zn₃N₂ nanoinclusions in N doped ZnO explaining the controversially discussed nitrogen related signals in the Raman spectra and why the incorporation of nitrogen as an acceptor is limited. For Cu₂O and SnO it is possible to identify the dominating native defects by RoRa in combination with a detailed group theory analysis. Such defects lead to the activation of forbidden Raman modes by symmetry reduction as well as influence optical and electronic properties. Our experimental results show that RoRa is a powerful technique to study defects and impurities whose knowledge is mandatory for making oxide semiconductors commercially feasible.

Oxide based low bandgap photoelectrocatalyst materials that comply with the energetic requirements for water splitting almost all suffer from low mobility and small carrier diffusion lengths which limits the photocatalytic performance [1]. Trapping and recombination in bulk and surface states account for the losses in most known metal oxides [2]. To increase the charge carrier lifetime in metal oxide photoelectrodes it is crucial to understand how material composition is related to carrier transport and capture dynamics over a wide time window. Our objective is to explore the influence of lattice defects, doping and surface effects on the recombination kinetics of Fe2O3, BiVO4 [3] and other materials by applying a set of complementary time-resolved laser spectroscopies that address different carrier processes under adapted different experimental conditions ranging from UHV to photoelectrochemical environment. THz spectroscopy (sub-ps to ns) and time-resolved microwave conductivity (ns to ms) probe the sample conductivity relating to charge carrier transport. For BiVO4 the transients show that doping reduces the carrier lifetime and temporal snapshots of THz mobility spectra taken after photoexcitation point to weakly-bound carriers with low mobility in agreement with an observed broad sub-bandgap luminescence band for the same samples. Ultrafast transient absorption (fs to ns) and a photo-induced absorption setup (ns-ms) probe the dynamics related to specific energetic states such as intra-band gap states that trap photoholes (Fe2O3). The latter technique is also applied to study the interfacial semiconductor dynamics and catalyst charge transfer kinetics in a working photoelectrochemical cell.
[1] A. J. Cowan, J. R. Durrant,
Chem. Soc. Rev. 42, 2281 (2013)
[2] A. G. Joly, J. R. Williams, S. A. Chambers, G. Xiong, W. P. Hess, D. M. Laman,
J. Appl. Phys. 99, 053521 (2006)
[3] F. F. Abdi, T. J. Savenije, M. May, B. Dam, R. van de Krol, J. Phys. Chem. Lett. 4, 2752 (2013)

Multiferroic heterostructures, which simultaneously exhibit ferroelectric and magnetic orders as well as the strong coupling between the two orders, have provided a wealth of scientific interest during the last decades. Recently, Pb(Mg1/3Nb2/3)0.7Ti0.3O3(PMN-PT)-based multiferroic heterostructures have attracted increasing research interest due to the large piezoelectric response and the diversified polarization switching paths. Here we reported an observation of a voltage-driven non-volatile, reversible, and deterministic magnetization reversal in a 10 nm NiFe film sputtered on a (110)-oriented PMN-PT layer using the magneto-optical Kerr microscopy (MOKE). During the measurement, no external magnetic field is applied and hence allows an in-situ observation of purely voltage-modulated magnetic domain changes. This voltage-driven magnetization reversal is attributed to a 90 degree magnetization switching induced by a metastable ferroelastic switching in rhombohedral ferroelectric, plus a tunable Dzyaloshinskii-Moryia (DM) interaction between the two layers. By manipulating the magnetic properties with an applied electric field, instead of the usual current or magnetic field, which could cause much less Joule heating in magnetic devices, the heterostructures provide a potential technological application of voltage-controlled magnetoresistive random access memory (MRAM).

Ce- and Bi-substituted yttrium iron garnets (Y3Fe5O12 / YIG) exhibit enhanced magneto-optical properties such as Faraday rotation and nonreciprocal phase shift (NRPS). These garnets have dominated the research in magneto-optical films for nonreciprocal photonics, imaging, recording, and spatial light modulators. However, monolithic integration of these thin films requires a two-step deposition method in which an ultrathin YIG seedlayer must be deposited and annealed prior to the deposition of the substituted garnet layer. Simulations of light travelling through garnet-cladded semiconductor waveguides showed that even a 20nm thin YIG seedlayer decreases the interaction of light with the high-NRPS substituted YIG layer by 40%. Fortunately, the garnet structure can accommodate half of the elements in the periodic table, including many interesting rare-earth/iron combinations. We present the study of Terbium Iron Garnet (Tb3Fe5O12 / TIG) that has properties yet unexploited by integrated photonics.
Here, TIG was grown without a seedlayer on double-side polished Si wafer using metallic targets via reactive sputtering with 10% oxygen flux in the Ar process gas. X-ray diffraction (XRD) indicated that single phase garnet was obtained after a rapid thermal anneal at 900°C for 2 min in oxygen. The magnetizations were measured using vibrating sample magnetometry (VSM) and the index of refraction and Faraday rotation were measured at 1545.2 nm. Using identical sputtering and annealing parameters, single phase Ce:YIG thin films were grown on YIG and TIG seedlayers. Faraday rotation of Ce:YIG grown on TIG was measured to be 33% higher than that of Ce:YIG grown on YIG seedlayer.
TIG has compensation temperature below room temperature. Therefore, at room temperature, TIG has a Faraday rotation in the same direction as Ce:YIG, which aligns opposite to the magnetic field. On the other hand, the compensation temperature of YIG is above room temperature. So, at room temperature, YIG has a Faraday rotation in the direction of the magnetic field (opposite to TIG and Ce:YIG). Thus, the NRPS of a waveguide with Ce:YIG would be enhanced by the TIG seedlayer and diminished by the YIG seedlayer. Finite Difference Time Domain (FDTD) simulations showed that a waveguide device fabricated with alternating garnet layers with opposite Faraday rotations enables “push/pull” optical effects for an overall reduced device footprint.

Atomically flat (111) interfaces between insulating perovskite oxides can provide a landscape for new electronic phenomena. For instance, the metallic ion / metal oxide bilayers that comprise the unit cell require both interfacial layers to be polar, generating an intrinsic polar discontinuityⁱ. The graphene-like coordination between interfacial metallic ion layer pairs provides alternate band structure leading to topologically protected states^ii,iii. We look at a specific example of an atomically smooth (111) heterointerface, in the model system of (111)-oriented LaAlO₃ / SrTiO₃ (LAO/STO) that preserves many features characteristic of exotic electronic interfacial states, and find that the LAO overlayer relieves the structural reconstruction of the STO (111) surface. The energetics are satisfied by global electronic reconstruction. This is confirmed by the measured transport characteristics of the resulting interfacial conducting layer, the direct determination of the structure and atomic charge distribution by Coherent Bragg Rod Analysis (COBRA), and theoretical calculations of electronic and structural characteristics. Interfacial behaviors of the kind discussed here will likely lead to new interfacial electronic devices.

SrTiO₃ is a well-known quantum paraelectric material to show a variety of fascinating properties by electron doping such as high electron mobility and superconductivity. In particular, two-dimensional electron systems (2DES) based on SrTiO₃, such as LaAlO₃/SrTiO₃ and SrTiO₃/n-SrTiO₃/SrTiO₃ delta-doped heterostructures, have attracted much attention since these systems exhibit two-dimensional superconductivity and Shubnikov-de Haas oscillations in the same host material [1]. However, electron mobility in these 2DESs is lower than that in bulk system because it is extremely sensitive to subtle lattice distortions caused by epitaxial strain or nonstoichiometry of the Sr/Ti ratio.
In this study, we aimed at improving the quality of the delta-doped SrTiO₃ system by using metal organic gas source molecular beam epitaxy (MOMBE) [2]. In this method, Sr and La metals were evaporated by effusion cells, where La atoms act as dopants by substituting the Sr sites. Titanium tetraisopropoxide was used as the Ti source, supplied by thermal evaporation without any carrier gas, while distilled pure ozone was employed as oxidant. The films were grown on SrTiO₃ (100) substrate under an ozone partial pressure of 1 × 10^-6 Torr at a substrate temperature of 1200 °C, which can be achieved with a laser heating system and is much higher than used in previous MOMBE [2]. To fill the oxygen vacancies formed during the growth, the samples were annealed in the growth chamber at 600 °C in P_ozone = 1 × 10^-6 Torr for one hour after deposition. The maximum electron mobility of uniformly La-doped films (300 nm thickness) grown under these conditions exceed 50,000 cm²/Vs with an electron density as low as 3.3 × 10¹⁷ cm^-3 at 2 K, which indicates extremely high crystalline quality and low impurity concentrations.
In order to form 2DES, SrTiO₃/La:SrTiO₃/SrTiO₃ structures were grown with various La-doped layer thicknesses to vary confinement potential. The 2DES exhibits the maximum electron mobility of 24,000 cm²/Vs, exceeding the maximum value of bulk crystal. Notably, low-temperature magnetotransport showed remarkably strong Shubnikov-de Haas oscillations, suggesting that parasitic three-dimensional components are substantially reduced compared with previous reports of LaAlO₃/SrTiO₃ or delta-doped structures [1,3] because of the sufficient confinement and low carrier density. Our result indicates that high-temperature growth is effective in MOMBE towards realizing complete two-dimensional states in SrTiO₃.
[1] A. Ohtomo, H. Y. Hwang, Nature 427, 423 (2004), N. Reyren et al., Science 317, 1196 (2007), Y. Kozuka et al., Nature 462, 487 (2009).
[2] J. Son et al., Nature Mater. 9, 482 (2009).
[3] B. Jalan et al., Phys. Rev. B 82, 081103(R) (2010).

There is a great deal of interest and excitement in the study of the quasi two-dimensional electron gas (q2DEG) confined at the interface between the two band insulators, SrTiO3 and LaAlO3 [1]. To further progress towards quantum and ballistic transport, it is essential to obtain samples of higher and higher mobility. Here we demonstrate a powerful and flexible approach to enhance the mobility of the q2DEG by remote control from the LaAlO3 surface using surface charges and adsorbates [2]. By optimizing the LaAlO3 growth and surface control, we enhance the Hall mobility of the q2DEG to >20,000 cm2V-1s-1. Interestingly, we find that despite the large variety in growth and surface control, there is a universal trend that mobility increases with decreasing sheet carrier density, shedding light on the microscopic scattering mechanisms of the interfacial electrons. Furthermore, the magneto-transport behavior of the q2DEG in a low sheet carrier density regime reveal two compelling features: 1) the presence of plateaus in the Hall conductivity that can be assigned to Landau filling indices with an interval close to 4 in units of e2/h (e is the electric charge and h is Planck&’s constant), implying a 4-fold degeneracy in the band structure; 2) a transition of the frequency of the Shubnikov-de Haas quantum oscillations (from f to ~3f) with increasing magnetic field. These experimental features can be explained by a magnetic breakdown transition, which quantitatively explains the structure and degeneracy of the measured Fermi surface.
This work is supported by the Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-76SF00515. Y.X. also acknowledges partial funding from the AFOSR-MURI on “Quantum Preservation, Simulation & Transfer in Oxide Nanostructures”.
[1] H. Y. Hwang et al., Nat. Mater. 11, 103 (2012).
[2] Y. Xie et al., Adv. Mater. 25, 4735 (2013).

Magnetic effects have been observed at the interface between two non-magnetic oxides SrTiO3 and LaAlO3 [1-3], which has sparked intense research interest of late. Currently, a number of theories have been proposed to explain this unexpected behaviour; however, to date, the origin of magnetism in this system remains controversial. We have investigated the magnetic properties of native vacancies in the LaAlO3 overlayer of the SrTiO3/LaAlO3 interface using first-principles density functional calculations, including energetics and electronic structure. We propose a new model accounting for ferromagnetism in this system and discuss our results in light of recent experimental observations.
[1] A. Brinkman et al., Nat. Mater. 6, 493 (2007).
[2] J. A. Bert, B. Kalisky, C. Bell, M. Kim, Y. Hikita, H. Y. Hwang, and K. A. Moler, Nat. Phys. 7, 767 (2011).
[3] L. Li, C. Richter, J. Mannhart, and R. C. Ashoori, Nat. Phys. 7, 762 (2011).

Transition metal oxides (TMO) offer various functionalities ranging from electronic devices to environmental catalysts [1, 2]. Often, the central part of such devices is the interface band alignments between different materials. In order to more flexibly design functional devices, techniques to control band alignments independently from their bulk properties are strongly desired. Originally developed in semiconductor heterostructures, the use of interface dipoles is based on the interface electrostatic boundary conditions and is one of the most fundamental methods to tune band alignments [3]. Given their strongly ionic nature and their accessibility to multiple valence states, TMO interfaces should be more suitable than covalent semiconductors for manipulating band alignments.
Here we focus on epitaxial metal-semiconductor Schottky interfaces between perovskite oxides to demonstrate the effectiveness of this technique. We will present two SrTiO3 based perovskite Schottky junctions (SrRuO3/SrTiO3 and La0.7Sr0.3MnO3/SrTiO3) in which the interface energy barriers were modulated by interface dipoles controlled on the atomic scale [4, 5]. Furthermore, we will introduce our recent spectroscopic results on how the strongly correlated nature of TMO influences the band alignments at manganite/titanate junctions suggesting new approaches to designing interface band structures.
1. R. Ramesh and D. G. Schlom, MRS Bull. 33, 1006 (2008).
2. J. Suntivich et al., Science 334, 1383 (2011).
3. F. Capasso et al., Appl. Phys. Lett. 46, 664 (1985).
4. Y. Hikita et al., Phys. Rev. B 79, 073101 (2009).
5. T. Yajima et al., Nature Mater. 10, 198 (2011).

The polar discontinuity at the SrTiO3/GdTiO3 (STO/GTO) interface [1] has been shown to provide a high concentration 2DEG [2]. However, It is a challenge to modulate such large charge densities. In this work we discuss the use of advanced energy band and device engineering to enable charge modulation of 10x1014 cm-2, which represents the highest 2D charge modulated in any semiconductor system. The ability to modulate such high charge density could enable a new class of devices that exploit the electronic, plasmonic, and correlated properties of extreme charge density oxide heterostructures.
The epitaxial structures investigated in this work were grown on (001)-oriented (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) single crystal substrates by molecular beam epitaxy (MBE) using a metal organic precursor (titanium isopropoxide) for Ti [3]. The basic structure consists of a GdTiO3/SrTiO3 structure, with STO on top. Due to the staggered band line-up between GTO and STO, a 2D electron gas channel with a sheet charge density of 3x1014 cm-2 is formed in the wider bandgap STO layer. This inverted STO/GTO HFET structure provides several advantages including large breakdown strength in the STO cap layer and a back-barrier for confinement.
We have used two approaches to increase charge modulation, which is limited largely by the low permittivity interfacial layer between metal and STO. The first approach uses a thick STO layer to reduce the relative effect of the interfacial layer. The second approach utilizes a new oxide-based design consisting of Ba0.5Sr0.5TiO3 (BST)/STO/GTO heterostructure field effect transistor layer, with the BST dielectric layer to reduce the gate leakage and improve gate capacitance. Detailed energy band diagrams and vertical transport for this new structure will be reported.
Sub-micron transistors with low contact resistance 0.5 Omega;.mm, as well as several test structures were used to understand the electronic properties of these structures. To study the effect of STO thickness we used three different structures - (28/40/98) nm STO/5 nm GTO/LSAT. From C-V measurements, we found higher charge modulation as the STO was increased, reaching 1.05x1014 cm-2 charge modulation. The use of the double heterostructure BST/STO/GTO enabled lower gate leakage and charge modulation of 8.7x1013 cm-2. Details of transistor characteristics, energy diagrams, field dependent dielectric properties, charge-dependent mobility, and high-field transport and velocity saturation in STO will be discussed in detail.
In conclusion, we demonstrated for the first time modulation of 2D carrier density greater than 1014 cm-2 in any material system. The work done here could enable fundamental studies of extreme electron concentration oxide interfaces, as well as new device applications based on these materials.
[1] Hwang et al. Nature Materials 11,103-113 (2012) [2] Moetakef et al. Appl. Phys. Lett. 99, 232116 (2011) [3] Jalan et al. J. Vac. Sci.Technol. A 27,461 (2009).;

At heterointerfaces in epitaxial films strain, chemistry, and symmetry can change, interact, and enable new functionalities. A paradigmatic case is the (001)-oriented LaAlO3/SrTiO3 (LAO/STO) heterostructure, where a confined two-dimensional electron liquid (2DEL) at the interface between these two oxide band-insulators is found. Yet, with the recently uncovered 2DELs for other growth orientations, (110)- and (111)- oriented LAO/STO interfaces [1,2], it has been realized that the response of such interfaces to built-in potentials may be more complex than previously thought, and that the polar discontinuity scenario might not be enough to account for the metallic state at the LAO/STO interface. We have used aberration corrected scanning transmission electron microscopy (STEM) in combination with electron energy loss spectroscopy (EELS) to probe in real space, with atomic resolution, how these interfaces counters the effect of existing potentials along different orientations. The mapping of the oxygen sub-lattice showed that the (110)-oriented interface, which in the ideal non-reconstructed limit should not have any polar discontinuity, evidence the absence of polar distortions across the 110 LAO/STO interface, in contrast to the distortions observed for the (001)-interface [3], where large electrostatic potentials appear because of the polar character of this interface. On the other hand, the (111)-interfaces appear to be much more diffuse and much less sharp than (001) and (110) orientations, showing an increased number of misfit dislocations, which in turn degrade their transport properties.
*Electron microscopy work at ORNL U.S. supported by the U.S. DOE-BES, Materials Sciences and Engineering Division and by ORNL&’s Center for Nanophase Materials Sciences (sponsored by DOE-BES). Research at UCM supported by the ERC Starting Investigator Award and Fundacioacute;n Caja Madrid.
[1] G. Herranz et al., Scientific Reports 2 758 (2012)
[2] A. Annadi et al., Nature communications 4 1838 (2013)
[3] C. Cantoni et al., Advanced Materials 24 3952 (2012)

In semiconductors, record high mobilities have been achieved in 2DEGs formed by modulation doping. However, in the 2DEG at the LaAlO3/SrTiO3 interface, electron mobilities remain stubbornly below those of bulk SrTiO3 crystals. We rule out two plausible hypotheses for the low mobility, ionized impurities and multiple subbands, based on our observation that mobility scales with carrier concentration to the -3 power. One hypothesis that remains plausible is that interface scattering limits mobility. We investigated this hypothesis by artificially doping the interface with magnetic and spin-orbit scatterers, Tm and Lu. At the 2% level, these dopants had no discernible effect on mobility or magnetoresistance from 2 K to 300 K. The mobility of doped samples were limited by phonons at high temperature and had upturns in resistivity at temperatures of 2 K to 10 K. At low temperatures, out-of-plane magnetoresistance was parabolic at low field and linear at high field. All out-of-plane magnetoresistance scaled with mu*B, in accordance with Kohler's rule, suggesting orbital effects. In contrast, in-plane magnetoresistance did not obey Kohler's rule - it ranged from -5% to -50% at 7 T. Understanding and controlling interfacial disorder at oxide interfaces remains a key challenge for this materials system as well as generally for the field of oxide interfaces and superlattices.

How the presence of a two-dimensional electron gas, formed at a ZnMgO/ZnO hetero-junction, shapes the corresponding low-field electron mobility, is explored. This analysis, cast within the framework of the model of Shur et al. [M. Shur et al., J. Electron. Mater. 25, 777 (1996)], accounts for the contributions to mobility related to the ionized impurity, polar optical phonon, piezoelectric, and acoustic deformation potential scattering processes, the overall mobility being evaluated through the application of Mathiessen&’s rule. A best fit, with the experimental results of Makino et al. [T. Makino et al., Appl. Phys. Lett. 87, 022101 (2005)], is obtained. A comparison, with results corresponding to other compound semiconductor hetero-junctions, is made. How the overall Hall factor is shaped by the presence of the two-dimensional electron gas is also considered. Finally, the device implications of these results are explored.

Interfaces of metal oxides to electrolytes are used in batteries and dye-sensitized solar cells to promote electrochemical charge transfer. In numerous applications like high charge density capacitors and electronic double layer (EDL) gated devices however, electrochemical changes in the oxide - electrolyte interface are to be avoided. A new class of electrolytes are ionic liquids. These organic salts are liquid at room temperature and exhibit vapor pressures in the range of nPa. Furthermore, in combining them with a polymer host matrix, they can be processed into gels and rubbery tapes while maintaining high EDL switching velocities and charge densities. Owing to their advantages, ionic liquids enjoy growing popularity as materials in electronic devices and for scientific analysis at high electrostatic fields. We demonstrate how ionic liquids can be used to draw conclusions about the morphology of metal oxides by probing the carrier mobility in the trap-filled limit. We show in situ biasing for a variety of characterization methods like low temperature conductivity, photoelectron spectroscopy, and Seebeck measurements. By analyzing the electrochemical stability and oxygen kinetics in nickelates and zinc oxide, we show the significance and limits of electrostatic doping in metal oxides.

Chemical methods of thin-film deposition are an affordable and versatile alternative to physical deposition techniques (PLD, Sputtering, MBE, etc.). Among them, deposition from a solution is preferred to avoid the use of high-vacuum chambers, and, a priori, is more suitable for coatings of large areas than physical and chemical vapor deposition processes. But when compared to physical techniques, chemical methods show poorest control of the thickness and stoichiometry, a larger interface and surface roughness, and difficult fabrication of electronically homogeneous films and multilayers over large areas (particularly for ultrathin films). These problems limited the applicability of chemical deposition methods for the study of the subtle phenomena that occur at clean interfaces with a broken symmetry (spin, orbital, etc), or their use in highly demanding applications.
Here I will describe a chemical deposition method to synthesize high quality ultrathin films (from 4nm to 50 nm) and multilayers, of different materials. The method uses aqueous solutions of environmentally friendly polymers and metallic salts. I will show that this technique allows the fabrication of homogeneous films, free of defects over large areas (several inch2), with smooth surfaces and interfaces, comparable to these obtained by physical methods. I will discuss the structural, magnetic and transport properties (Hall, electrical conductivity and Seebeck coefficient) of some oxide layers and multilayers sythesized by this method. These include misfit oxide cobaltates with thermoelectric properties, LaCoO3/LaSrMnO3 bilayers, etc.

Pulsed Laser Deposition (PLD) has been established in recent years as a versatile thin film deposition technique for complex materials, including complex oxides. The growth process of near stoichiometric ablation and transfer of material, and supersaturation on the substrates surface contribute to a near stoichiometric synthesis of complex transition metal oxides, which in turn exhibit a large variety of interesting physical properties, such as high temperature superconductivity and many ferroic properties.
These properties are highly sensitive to slight deviation from ideal stoichiometry. Studies on the synthesis of superconducting Sr2RuO4 have shown this narrow window of growth parameter settings to achieve perfect stoichiometric growth[1]. Also the occurance of the 2-dimension electron gas at the LaAlO3/SrTiO3 interface and its dependence on the synthesis parameters hasn&’t been fully understood yet[2].
Many oxide material that have become of great interest in recent years for there electronic properties contain volatile species such as ruthenium, lead and bismuth, were the volatility, and hence, different sticking mechanisms of these species impede a stoichiometric synthesis. Here, we present a fundamental investigation on the influence of growth pressure on plasma composition, kinetics, which subsequently influence particle substrate interaction, particle sticking and diffusion. Insight on key mechanisms involved in optimized growth conditions will eventually lead to full control over stoichiometry, doping, defect density of PLD thin films and improved thin film properties.
We use Optical Emission Spectroscopy (OES), supported with Laser Induced Fluorescence (LIF) measurements, which gives a unique overview on the quantitative elemental distribution in the plume. It is shown that SrTiO3 film stoichiometry strongly depends on background pressure, where in a relative small pressure range, from 0.01 mbar to 0.1 mbar oxygen pressure, often optimal stoichiometry has been observed, for example, as determined with X-Ray Diffraction (XRD). In this pressure regime, OES and LIF measurements show that a transition occurs from incomplete to full oxidation of species. This suggests that the oxidation of species of the plasma is a crucial mechanism in the stoichiometric reconstruction of the synthesized oxide thin films.
[1] Krockenberger et al. Applied Physics Letters, 97(8), (2010).
[2] Aruta, C. et al. Applied Physics Letters, 97(25(2010)
Perspective: Building on transient plasmas (BTP), STW proposal 10760

Perovskite oxide thin films and their particular electrical characteristics have been widely investigated for the micro-electronic devices. However, conventional processes available for the fabrication of perovskite oxide thin films are not easily compatible with the manufacture of current flexible electronics, because they require high processing temperatures (>600°C) to obtain the perovskite phase. In this works, the amorphous perovskite thin films such as Pb(Zr,Ti)O₃, BaTiO₃, (La,Sr)MnO₃ were prepared on the polyimide substrate, and crystallization route of the amorphous perovskite oxide thin films are investigated. Excimer laser annealing (ELA) as a promising heat treatment was used for low temperature crystallization route of perovskite thin films. To determine the experimental condition of excimer laser annealing, relation between irradiated laser energy and temperature distribution in perovskite thin films was expected using COMSOL with a heat transfer module. The structural and electrical characteristics of the excimer laser cryatallized perovskite thin films were evaluated.

With the strong trend to size reduction in the oxide systems and the reliance of device functionalities on the properties and quality of interfaces, understanding film growth dynamics and formation of surface structures at the atomic level is of critical importance for development of new devices and technologies for electronic, magnetic, energy and other applications as well as for interpretation of experimental data. Here, we report atomically resolved in-situ Scanning Tunneling Microscopy (STM) studies of Pulsed Laser Deposition (PLD)-grown thin films of La_5/8Ca_3/8MnO₃ (LCMO). LCMO is a widely studied perovskite manganite, owing to its colossal magnetoresistance and semiconductor-to-metal transition. Films were grown on TiO₂-terminated (001) SrTiO₃ substrates at a temperature of 750 °C and oxygen pressure of 50 mTorr. Reflective High Energy Electron Diffraction (RHEED) was used to monitor the growth in-situ. In-situ STM measurements were performed at room temperature. Atomic resolution was obtained for all investigated samples. LCMO is known to grow in layer-by layer (LBL) mode. We find that the initial growth does not follow the best physically possible LBL growth (with only three u. c. layers exposed to the film surface). This behavior is reflected in decaying oscillations of the RHEED intensity at the specular spot during deposition of the first 10-15 unit cells. Subsequently, the RHEED intensity oscillations grow and remain persistent. In accordance with the RHEED behavior, STM images of 16 u. c.-thick films revealed surfaces with up to five u.c. layers being exposed in a stepped island-like morphology with ½ u. c. step heights. Such morphology allowed studies of atomic surface structure of both terminations; however, the terminations could not be identified. In turn, 25-u.c. thick samples were found to be almost single-terminated with the second (minor) termination present as island as well as being exposed through holes in the dominating termination. Atomically resolved images show that the minor terminations is ordered and exhibits (1x1) reconstructions with respect to the LCMO lattice, and RHEED data suggests that this termination is the A (La/Ca-O) termination. A 250 u. c.-thick film was found to be single-terminated with only three u. c. layers exposed, which indicates the best physically attainable LBL growth. Ex-situ electrical transport and XRD measurements indicated that the 250 u.c.-thick film is fully strained to the substrate with a semiconductor-to-metal transition at ~80 K, while in 25 u.c.-thick films, the transition is entirely suppressed down to 4.2 K.
This research was sponsored by the Division of Materials Sciences and Engineering (AT, RKV, SVK) and by the Scientific User Facilities Division (LQ, MDB, APB) of BES, DOE. Research was conducted at the CNMS, which is sponsored at ORNL by the Scientific User Facilities Division, BES, DOE.

A phase field model plus an efficient multi-scale simulation scheme is developed that solves transport of charged ions subject to interfacial reactions and electrostatic interaction. Without a priori assumptions such as zero electric current or local charge neutrality, the model directly solves the long range electric field according to the time-dependent charge distribution. Application of the model sheds some light on electric field effect on mass transport at various growth stages. The capability of the multi-scale scheme is demonstrated in simulating oxidation kinetics in a prototype metal system that spans several orders of magnitude in film thickness and exposure time capturing a continuous linear to parabolic transition of oxidation rate with growing film thickness. The possible influence of surface charge is also discussed which combines with space charge to determine effective ion diffusivities at thin film stage. This phase field model offers opportunities to quantitatively investigate microstructural effects with characteristic size comparable with the Debye length.

Recent studies of 5d transition-metal oxides have revealed exotic physics with strong potential for electronic device applications. Although weak electronic correlations and paramagnetism were conventionally anticipated to govern the physics of 5d transition-metal oxides owing to the extended nature of 5d electrons, recent research on Sr₂IrO₄ has shown that novel J_eff = 1/2 Mott states appear due to strong spin-orbit interactions. The energy scale associated with the spin-orbit coupling is comparable to the crystal-field energy and the on-site Coulomb interaction. The strong competition between these fundamental interactions creates the potential for the emergence of unprecedented electronic states, and it has been theoretically proposed that strong spin-orbit coupling can yield novel phases such as Weyl semimetals.
To understand the physics of Sr₂IrO₄ and to find a way of tuning its multiple, competing interactions, we have investigated epitaxial thin-films under various lattice-strain conditions, which is a viable alternative for creating desired modifications of the physical properties of the system. In this talk, we discuss the growth, transport, and optical properties of both c-axis and a-axis oriented Sr₂IrO₄ epitaxial thin films grown on various oxide substrates. The in-plane lattice mismatches between Sr₂IrO₄ and various oxide substrates can exert both tensile (+) and compressive (-) strain to films. Under tensile (compressive) strain, increased (decreased) Ir-O-Ir bond-angles are expected to result in increased (decreased) electronic bandwidths. However, the films under various strain have little change in their transport properties and show no clear correlation with applied strain. In optical spectroscopic measurements, we have observed that two optical absorption peaks near 0.5 eV and 1.0 eV are shifted to higher (lower) energies under tensile (compressive) strain, indicating that the electronic-correlation energy is affected by in-plane lattice-strain and interlayer-spacing. Our observations strongly suggest that not only the electronic bandwidth, but also the magnitude of the strong electronic correlation energy can be manipulated by lattice strain. The effective tuning of the electronic structure under lattice-modification provides an important insight into the physics driven by the coexistence of strong spin-orbit coupling and electronic correlation.

The 5d transition metal oxides have drawn much attention for their exotic phases arising from the interplay of the strong spin-orbit interaction and the electronic correlation [1]. The iridates are particularly interesting due to the comparable strength of their spin-orbit interaction and on-site electron-electron correlation. One system of recent experimental investigation is the Ruddlesden-Popper series Sr_n+1Ir_nO_3n+1 whose electronic structure is tunable from a two-dimensional insulating state in Sr₂IrO₄ (n = 1) to a three-dimensional correlated metallic state in SrIrO₃ (n = infin;) [2]. The mechanism of the metal-insulator transition in this system is currently contested, as a Slater transition has also been offered as a plausible explanation for the insulating property in Sr₂IrO₄ [3]. The contested role of the strong spin-orbit coupling in these iridates can be better understood by exploring the nature of metal-insulator transitions in the semi-metallic SrIrO₃.
In this presentation, we will discuss our experimental investigation of the electronic properties of epitaxially strained SrIrO₃ thin-films. The orthorhombic perovskite crystal phase of SrIrO₃ is synthesized as a thin film (~ 20 nm) on various substrates of (LaAlO₃)_0.3-(Sr₂AlTaO₆)_0.7, SrTiO₃, GdScO₃, and MgO using pulsed laser deposition. The correct perovskite phase and lattice strain of the SrIrO₃ thin-films are verified by x-ray diffraction and reciprocal space maps. Comparing the compressive to tensile strained samples of SrIrO₃, the in-plane lattice parameters of SrIrO₃ vary by as much as 1.7 % while the out-of-plane lattice parameter remains relatively constant—indicating in-plane octahedral rotation is more easily achieved with biaxial strain than out-of-plane expansion. Temperature-dependent resistivity data indicate a strong dependence on lattice strain. We have observed that when the in-plane lattice parameters are tuned from tensile to compressive strain, the electronic behavior of the strained SrIrO₃ thin-films changes from metallic to insulating. All samples have sheet resistance below 13 kOmega;/sq, which is far below the 25 kOmega;/sq crossover between weak and strong localization [4]. The insulating samples were fit using the Mott variable-range-hopping equation at low temperatures, (< 15 K), which is believed to be the conducing mechanism of Anderson localization at finite temperature [4]. The strain-dependent metal-insulator transition in epitaxial perovskite SrIrO₃ thin-films offers an important insight into the electronic band structure of these strongly spin-orbit coupled materials and provides a foundation for fabricating more elaborate 5d transition metal oxide heterostructures.
[1] D. A. Pesin and L. Balents, Nature Phys. 6, 376 (2010).
[2] S. J. Moon et al., Phys. Rev. Lett. 101, 226402 (2011).
[3] R. Arita et al., Phys. Rev. Lett. 108, 086403 (2012).
[4] R. Scherwitzl et al., Phys. Rev. Lett. 106, 246403 (2011).

Structural and electrical properties of an oxygen-deficient SrFeO2.875 thin film were investigated. The film was prepared by ex-situ oxidizing annealing of a brownmillerite SrFeO2.5 film deposited on a (001) SrTiO3 substrate by pulsed laser deposition, and was confirmed to be an epitaxially grown single phase. SrFeO2.875 contained an unusual high-oxidation state of Fe, Fe3.75+, and the oxygen vacancies in the compound were disordered at about 650 K. When the SrFeO2.875 thin film was cooled from the high temperature, a structural phase transition due to oxygen-vacancy ordering occurred at 620 K. Electrical resistivity drastically increased, and a metal-insulator transition was observed at the same temperature. Mossbauer spectroscopy revealed that the SrFeO2.875 thin film showed charge disproportionation from Fe3.75+ to Fe3+ and Fe4+. The behaviors were in sharp contrast to the ones observed in a bulk sample, which showed the structural phase transition at 600 K followed by the charge disproportionation transition at 70 K. It is thus concluded that the charge disproportionation transition temperature in the present thin film sample drastically increased from 70 K to 620 K due to the epitaxial strain effect.

Rare earth titanates (RTiO₃, where R is a trivalent rare earth ion) are prototypical Mott insulators. The properties of these materials are controlled to a large extent by oxygen octahedral tilts in the crystal structure, which is derived from the cubic perovskite but orthorhombically distorted. The tilts systematically depend on the rare earth ion size, and increase from La to Y. GdTiO₃ is one of the rare earth titanates with ferromagnetic properties below 30 K. SrTiO₃ is a diamagnetic band insulator. GdTiO₃ can be hole doped by substituting Gd with Sr ions. At a critical Sr concentration an insulator-to-metal transition occurs. SrTiO₃ can be electron doped by Gd. In solid solutions of Gd_xSr_1-xTiO₃, transition from hole to electron conduction and suppression of the ferrimagnetism is expected as the Sr content increases. Although these studies have performed for other rare earth titantes in bulk form, understanding of the underlying physics of transport requires study of stoichiometric and defect-free material. In this work, stoichiometric and epitaxial Gd_xSr_1-xTiO₃ thin films were grown using hybrid molecular beam epitaxy on SrTiO₃ (for x le; 0.05) and (La_0.3Sr_0.7)(Al_0.65Ta_0.35)O₃ (for x > 0.05) single crystal substrates, and were investigated for electrical transport and magnetic properties. Electrical characterization revealed a metal-insulator transition at a composition of 0.65 < x < 0.8, which is accompanied by a transition from p-type to n-type conduction (as observed in thermoelectric analysis). These observations are in good agreement with theory and results from other rare earth titanates in the literature. Metallic compositions showed Fermi-liquid characteristics indicating the importance of electron-electron interactions. The temperature-dependent resistivity of GdTiO₃ films is best described by formation of small polarons, while the temperature dependent resistivity of insulating Gd_xSr_1-xTiO₃ films were best described by small polaron hopping in a disordered system. The decrease in the activation energy with increase in Sr concentration supported the increase in polaron size as the insulator-metal transition is approached. Furthermore, suppression of magnetism was observed as the concentration of Sr is increased from zero in GdTiO₃ to the composition of the metal-insulator transition.

In transition-metal oxides, the epitaxial strain arising from a lattice mismatch between the film and the underlying substrate provides an opportunity to enhance the material functionality and design novel physical properties that are different from their bulk counterpart.[1] It is known that biaxial misfit-strain could be accommodated by one or combination of following mechanisms:[2] (i) change of the bond length between the cation and oxygen and out-of-plane lattice parameter according to Possion&’s ratio, (ii) rotation and tilting of transition-metal-centered/corner-sharing octahedra, often changing crystallographic symmetry, electronic, and magnetic properties, and (iii) formation of point defects. In particular, for in-plane tensile strain, oxygen vacancies can be easily created with the expansion of the unit-cell volume. Change of the oxygen stoichiometry in rare-earth nickelates affects a wide range of the physical properties including conductivity and ferroic order.
In this work, we demonstrate oxygen-vacancy-mediated strain stabilization in epitaxial rare-earth nickelate (001) thin films. Experimentally, we find that the volume of samples grown under in-plane compressive strain is roughly constant, whereas it increases for those grown under in-plane tensile strain. Our transport measurements show that the samples grown under compressive strain more conductive compared to those grown under tensile strain. Our density-functional calculations predict that the oxygen vacancy formation energy decreases as a function of in-plane tensile strain. It is found that rare-earth nickelates are perfectly stoichiometric under compressive strain; however, they are reduced slowly to become non-stoichiometric under tensile strain, indicating that the formation of oxygen vacancies is crucial for stabilizing strain in rare-earth nickelate films. This mechanism also explains the behavior of electrical conductivity. Thus, our work presents the controllability of phase stoichiometry in rare-earth nickelates via strain engineering.
[1] D. G. Schlom et al., Annu. Rev. Mater. Res. 37, 589 (2007).
[2] U. Aschauer et al., Phys. Rev. B 88, 054111 (2013).

SrRuO3 is an itinerant ferromagnet that has generated a large amount of interest due to its potential use as an electrode layer in complex oxide heterostructures. By applying epitaxial strain on ultra-thin films of SrRuO3 one can essentially tune the structure and properties. Substrate induced strain is commonly used to alter its physical properties, effectively by adjusting the structural parameters, i.e. bond distances and RuO6 octahedral tilts and rotations. We provide experimental evidence for an enhancement of the ferromagnetic Currie temperature by the addition of a capping layer. We propose this enhancement is due to capping-layer-induced strain relief or tilting of the oxygen octahedra, agreeing well with DFT calculations. The model is supported by density functional calculations. Using a scanning Sagnac magneto-optic microscope we demonstrate the spatially controlled modification of magnetization. This work demonstrates a new route to local control of magnetism in complex oxides based electronic device architectures.

Since the discovery of the spin Seebeck effect (SSE) much attention has been devoted to the study of the interaction between heat, spin and charge in magnetic systems, the SSE refers to the generation of a spin voltage upon the application of a thermal gradient, which is detected by means of the inverse spin Hall effect (ISHE). Unlike the conventional Seebeck effect which requires the presence of electronic carriers, the SSE has also been observed in insulating magnetic materials, opening the possibility to explore magnetic oxides as potential candidates for thermoelectric applications. Therefore the study of the thermomagnetic transport properties in this type of materials is of interest in order to gain insight in the spin-heat interaction in these systems.
Here we report the anomalous Nernst (ANE) and SSE in magnetic oxides with a metal to insulator phase transition (i.e. magnetite, magnetically doped perovskiteshellip;). These studies are interesting to explore the effect of variations in the charge, lattice or orbital degrees of freedom on the spin transport in magnetic systems.

Interfaces of complex oxides materials provide a rich playground for the exploration of novel properties not found in the bulk constituents but also for the development of functional interfaces to be incorporated into applications. With recent advances in thin film deposition techniques, emergent phenomena at perovskite oxide interfaces have been studied intensively in order to understand how mismatches in bands, valences, and interaction lengths give rise to novel interfacial ground states. Surprisingly, there have been only a handful of successful efforts demonstrating new magnetic ground states at interfaces. In this talk, I will describe a recent example of our work demonstrating the generation of long-range ferromagnetic order at CaMnO₃ interfaces. We demonstrate ferromagnetism in superlattices composed of the antiferromagnetic insulator CaMnO₃ and an itinerant metal. We have performed experiments on CaMnO₃ /CaRuO₃ and CaMnO₃ /LaNiO₃ superlattices and have found that the ferromagnetism is confined to one unit cell as theoretically predicted. Moreover LaNiO₃ exhibits a thickness dependent metal-insulator transition and we find the emergence of ferromagnetism to be coincident with the conducting state of LaNiO₃ in CaMnO₃ /LaNiO₃ superlattices. That is, only superlattices in which the LaNiO3 layers are metallic exhibit ferromagnetism. Together these results suggest that the interface ferromagnetism in these CaMnO₃ superlattices are attributed to a double exchange interaction among Mn ions mediated by the adjacent itinerant metal.

Spintronics integrates electron spin into electronic devices, yielding novel functionalities for logic, memory, sensing and optoelectronics [1]. Many spintronic effects arise from parallel (P) and antiparallel (AP) configurations of adjacent ferromagnetic electrodes [1,2], where large signals require access to a fully AP state. Commercial devices engineer the AP state via an antiferromagnetic exchange bias layer [3], while lateral devices [4] generate coercivity contrast via shape anisotropy. The latter may produce a gradual switching that attenuates spintronic signals, so control of electrode micromagnetics is crucial. Here we explore this issue in epitaxial thin-film electrodes, whose switching can be tuned by crystal orientation, strain, thickness, composition or temperature.
We have studied patterned La_0.67Sr_0.33MnO₃ (LSMO), a popular electrode with high spin polarisation for spintronics based on oxide [5], multiferroic [6] and organic [7] materials. Varying the crystal orientation of the substrate dramatically affects the micromagnetic behaviour observed using photoelectron emission microscopy (PEEM) with X-ray magnetic dichroism (XMCD) contrast. Magnetometry on unpatterned films shows that the critical fields for nucleation and propagation of magnetic domain walls vary independently, leading to either sharp stochastic switching or gradual switching via polydomain states. With the aid of micromagnetic simulations, we relate the observed switching behaviour to the underlying magnetic anisotropy, and we provide suggestions for optimal electrode design.
[1] Zutic et al., Rev. Mod. Phys. 76, 323 (2004)
[2] Fert et al., IEEE Trans. Electron Dev. 54, 921 (2007)
[3] Parkin et al., Proc. IEEE 91, 661 (2003)
[4] Hueso et al., Nature 445, 410 (2007)
[5] Bowen et al., Appl. Phys. Lett. 82, 233 (2003)
[6] Garcia et al., Science 327, 1106 (2010)
[7] Xiong et al., Nature 427, 821 (2004)

Perovskite oxides exhibit a wide range of functional properties which may potentially be applied to next generation spintronic devices. Due to the strong correlation between charge, orbital, spin, and lattice degrees of freedom, the competing interactions at perovskite interfaces can result in novel phenomena that are absent in bulk materials. Exchange coupling between ferromagnetic (FM) and antiferromagnetic (AFM) layers has previously been investigated in (100)-oriented FM La0.7Sr0.3MnO3 (LSMO) / G-type AFM La0.7Sr0.3FeO3 (LSFO) superlattices. Due to the spin frustration at the interface, spin-flop coupling occurs characterized by an in-plane perpendicular alignment between the LSMO moments and the LSFO spin axis. [1][2][3] In contrast, because exchange coupling is determined by the interfacial spin configuration of the AFM layer, (111)-oriented LSMO/LSFO superlattices are expected to display exchange bias due to the presence of uncompensated spins. In this work, we used soft X-ray magnetic spectroscopy and photoemission electron microscopy to compare the exchange coupling in (100)- and (111)-oriented superlattices. We find that in the (111)-oriented superlattices the spin axis of LSFO cants out of the plane of the sample, while it lies in-plane for a LSFO single layer and a LSMO/LSFO bilayer. This behavior is in direct contrast to the in-plane spin axis of LSFO in the (100)-oriented superlattices. [2] Both orientations display a correlation between the FM and AFM domain patterns and the AFM moments can be reoriented by an external magnetic field. Therefore, spin-flop coupling appears to be very stable in the LSMO/LSFO superlattices regardless of film orientation due to the combined effects related to AFM spin configuration, crystallographic symmetry, and charge transfer.
[1] Y. Takamura et al., Phys. Rev. B 80, 180417(R) (2009)
[2] E. Arenholz et al, Appl. Phys. Lett. 94, 072503 (2009)
[3] Fan Yang et al, Phys. Rev. B 83, 014417(R) (2011)

The interface of complex oxide is a rare place where interactions among intrinsic ordering parameters of spin, orbital, and charge can couple with extrinsic modifications induced by epitaxial strain, doping and defects to produce unique physical phenomenon and materials properties. A typical example is the stabilization of a ferromagnetic ordering in thicker epitaxial LaCoO3 film when grown on SrTiO3 or LSAT substrates. These results are typically explained with a tensile strain induced low-spin to high spin or intermediate spin transition in the cobaltite.
Here we report some unusual structural and magnetic properties for thin LaCoO3 epitaxial films from 5 to 50 unit cells (uc) grown on SrTiO3. It is found that films thicker than 15 uc exhibit very similar transition temperatures (Tc ~ 80 K) and saturation moments (~ 0.65 mu;B/Co at 5 K), consistent with literature data. However, dramatic suppression of the spin transition is evidenced in ultra-thin films of 10 and 5 uc, in which the ferromagnetic ordering almost vanishes, suggesting the existence of a magnetic dead layer at this heterojunction. X-ray reciprocal space mapping around substrate {204} Bragg peaks reveals that for 15 uc film, there is a consistent film peak shift along the L direction when rotating from SrTiO3 (204) to (024), (-204) and (0-24) azimuths, which is absent for 5 uc film, indicating a breaking of tetragonal symmetry for 15 uc film. The structural results are supported by direct octahedral tilt imaging with quantitative scanning transmission electron microscopy, which reveals a coherent tetragonal lattice for 5 uc film and a significant oxygen octahedral tilt that eventually saturates to the bulk value of ~ 10° for 15 uc film. Density functional theory calculations attribute the observed deteriorated magnetic properties for ultra-thin LaCoO3 to the suppression of octahedral tilts, which modify the orbital degeneracy and exchange splitting inside the Co 3d orbitals. This is further supported by the fact that the magnetic property and octahedral tilt structure of a [SrTiO3(5uc)/LaCoO3(5uc)]3 superlattice are just in between those of single 5uc and 15 uc LaCoO3 films. These results highlight the significance of octahedral distortion in controlling the magnetic properties of complex oxides and provide new insights into the origin of magnetic dead layers at complex oxide heterojunctions.
This research was conducted at Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. DOE, at Oak Ridge National Laboratory.

CaMnO₃ (CMO) is a model system for studying the effects of epitaxial strain and interfacial engineering. Bulk CMO is an insulating antiferromagnet (T_N ~125 K) with weak ferromagnetism (FM) that is highly sensitive to electronic defects such as electron dopants and oxygen vacancies [1][2]. A recent DFT study suggests that CMO is similarly sensitive to epitaxial tensile strain through modifications to octahedral rotations and oxygen stoichiometry [3], both of which are known to affect magnetic properties. Despite such sensitivity, we have found that CaMnO₃ incorporated into heterostructures with an itinerant metal gives rise to interfacial FM as theoretically explained by Nanda et al [4]. Through careful study of CMO thin films and superlattices (SLs) of LaNiO₃ (LNO)/CMO, we have demonstrated that the FM in the heterostructures is due to a double exchange (DE) interaction of Mn at the CMO/LNO interfaces.
More specifically, we grew CMO thin films 20 to 75 nm thick on LaAlO₃ substrates that are coherently strained 1.6% in tension according to x-ray diffraction. X-ray absorption spectroscopy and Rutherford backscattering spectrometry confirm the stoichiometry CMO. SQUID magnetometry measurements indicate a magnetic transition temperature at ~95 K and no hysteretic behavior of magnetization with applied field. X-ray magnetic circular dichroism signal was not observed at the Mn L edge, thus suggesting that the 95 K transition temperature is associated with antiferromagnetism.
CMO was then incorporated into LNO/CMO SLs in which the origin of FM is thought to be a ferromagnetic DE interaction among interfacial Mn ions mediated by itinerant electrons. LNO undergoes a metal-insulator transition at a thickness of 3-4 unit cells which tests the role of itinerant electrons. We fabricated [(LNO)_m/(CMO)_n]₈ SLs with m=2-9 and n=8, 14, and 20. The emergence of FM coincides with the onset of metallicity in the LNO layer at m ge; 4. SLs with m < 4 are insulating and non-ferromagnetic. Therefore, neither epitaxial strain nor interfaces with a paramagnetic insulator are sufficient to induce FM in CMO. Electron itinerancy in LNO is required for FM in CMO heterostructures. PNR measurements confirm the FM to be confined to one CMO unit cell at the interface. Unexpectedly, we find a modulation in the saturated magnetic moment with LNO layer thickness. SLs with m=odd and >4 (5,7,9) have a significantly higher saturated magnetic moment than SLs with m=even and ge;4 (4,6,8). To investigate the nature of the oscillatory coupling in LNO/CMO SLs, neutron diffraction measurements were performed. Analysis of the CMO antiferromagnetic structure and correlation length in these SLs shed light onto the possible origin of this coupling.
1. I. Gil de Muro et al., J. Solid State Chem. 178, 14020-14023 (2005).
2. J. J. Neumeier and J. L. Cohn, Phys. Rev. B 61, 14319-14322 (2000).
3. U. Aschauer et al., Phys. Rev. B 88, 054111 (2013).
4. B. Nanda et al., Phys. Rev. Lett. 98, 216804 (2007).

It is well known that elastic couplings mediate long-range interactions between local degrees of freedom in colossal magnetoresistance manganites. Though the effects of elastic strain couplings on phase transitions have been extensively studied in the past [1], several important questions remain such as whether strain can induce inhomogeneous ordered states as those observed in manganites. In this talk, we address this question phenomenologically and propose that the observed scaling of the metal-to-insulator transition temperature on ionic radii in perovskite manganites [2] is the result of rotations of the MnO₆ octahedra that screen the strain fields.
Work at Argonne National Laboratory is supported by the U.S. Department of Energy, Office of Basic Energy Sciences under contract no. DE-AC02-06CH11357.
[1] D. J. Bergman and B. I. Halperin, Phys. Rev. B 13 2145 (1976).
[2] L. M. Rodriguez-Martinez and J. P. Attfield, Phys. Rev. B 54 R15622 (1996).

Structural and electronic properties of oxide surfaces control their physical functionalities and electrocatalytic activity, and are currently of interest for energy generation and storage applications. In this presentation, I will discuss several examples of high-resolution studies of the electronic and electrochemical properties of oxide surfaces enabled by combination of the in-situ Pulsed Laser Deposition growth with atomic resolution Scanning Tunneling Microscopy and Spectroscopy. For SrRuO3, we directly observe multiple surface reconstructions and link these to the metal-insulator transitions as ascertained by UPS methods. On LaxCa1-xMnO3, we demonstrate strong termination dependence of electronic properties and presence of disordered oxygen ad-atoms. The growth dynamics and surface terminations of these films are discussed, along with single-atom electrochemistry experiments performed by STM. These studies point towards local STM-based control of the order parameter that will enable much greater insight into structure and conductivity on as-grown thin film samples. We further extend the applicability of local crystallographic mapping to scanning tunneling microscopy data, allowing for mapping surface electrochemistry and order parameter fields on the atomic level in manganites and ruthenates.
This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences (BES), Materials Sciences and Engineering Division (SVK, AT, RV). AB is supported by the Center for Nanophase Materials Sciences which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy

The wide interest in characterizing iron oxide systems and their properties arises from the variety of their technological applications, , ranging from magnetic devices to heterogeneous catalysis [1]. The most common and stable iron oxides phases are Fe3O4 and alpha- Fe2O3; their crystal structure, stoichiometry as well as their surface properties can be tuned by special preparation procedures [2]. Moreover, in thin film systems the interaction with the substrate can strongly change the chemical and electric properties compared to the expected for bulk materials in oxidation/reduction reactions.
Relating to the application in catalysis, these two oxides are successfully used as support for noble metal nano-particle; furthermore, in some particular cases, the interaction between the nano-particle and the support (SMSI) can induce their encapsulation, affecting strongly the activity of the system. For instance, Pt nano-particles supported on Fe3O4 films are encapsulated by an FeO skin. This thermally activated process enhances the activity in CO oxidation under reaction conditions. [3][5]Also, significant properties of the system, like nano-particle size, density and shape are strongly affected by preparation parameters, such as the amount of deposited material, deposition and annealing temperature.
In this work we studied the growth and the stability of Fe3O4 and Fe2O3 thin films on a Pt(111) substrate, as well as their interaction with Pt nanoparticles. The investigations were carried out with a special spectromicroscope, the energy-filtered and aberration corrected LEEM-PEEM microscope, called SMART [6]. This multi-method instrument combines different spectroscopy, microscopy and diffraction techniques and operates at BESSY-2, the soft x-ray light source of the Helmholtz Center in Berlin, with a lateral resolution of 2.6 nm in LEEM and 18 nm in energy filtered XPEEM, which can only be obtained by simultaneous correction of spherical and chromatic aberrations. In addition, the high acquisition rate allows the in real time and in situ observation of surface processes like growth, chemical reaction and phase transition.
Combining LEEM/LEED and XPEEM we studied the oxidation and reduction mechanisms of mixed-phase iron oxide films, pointing out the substrate role in such reactions. Moreover, the influence of deposition temperature and oxygen atmosphere on the formation of Pt nano-particles on these iron oxide supports will be discussed.

The novel electronic transport, magnetic properties, and other exotic physical phenomena observed in correlated-electron “metallic” oxide (MO) materials have been extensively explored; however, the potential of these materials for energy applications has only begun to be investigated. In this work, we explore the observation and physics of novel light absorption and photovoltaic and photocatalytic activity in heterojunctions based on such MO materials.^1,2 Using pulsed-laser deposition, heterostructures based on a correlated MO [LaNiO₃, SrRuO₃, SrVO₃, La_0.5Sr_0.5CoO3 (LSCO) or La_0.7Sr_0.3MnO₃ (LSMO)] and TiO₂ were synthesized. The resulting heterostructures exhibit high-performance photocatalytic response and operate on the principle of hot carrier injection from the MO into the TiO₂. Extensive structural and physical property characterization (including temperature- and field-dependent resistivity and spectroscopic ellipsometry studies) were completed. The absorption coefficient (α) of the MO materials are large (>10⁵ cm^-1) and variations in α with photon energy are explained by exploring the electronic density of states. 50 nm thick films of these materials have low reflectance (17-33%) and high absorption (33-55%) of AM1.5G light which is comparable to designer direct band-gap semiconductors. Study of ITO/TiO2/MO heterojunctions reveals photovoltaic response in which the MO serves as the primary light absorber. Detailed dark current-voltage studies enable the construction of proposed band diagrams for these heterojunctions and reveal three types of devices: n-n ohmic (based on SrVO₃), n-n Schottky (based on SrRuO₃ and LaNiO₃), and p-n Schottky (based on LSCO and LSMO). All the Schottky devices displayed an exponential increase in short-circuit current density with photon energy indicating that the photovoltaic response arises from hot carrier injection from the MO into the TiO₂. Proposed band diagrams for photocatalytic TiO₂/MO heterojunctions are used to interpret the results of methylene blue degradation experiments. All the heterojunctions have activities at least 2X higher than the widely studied TiO₂ nanopowder, Degussa P25, with the LSCO-based device having an activity more than 26X higher. We will also probe the effects of epitaxial strain on the evolution of light-absorption in the MO materials, the role of compositional doping to vary the work function in the MO materials, and the effect of varying carrier concentration in the TiO₂ to expand our abilities to control the nature of properties and device performance.
1) S. Lee, et al., Adv. Energy Mater.3, 1084 (2013).
2) B. A. Apgar, et. al., Adv. Mater. (2013). DOI: 10.1002/adma.201303144.

While a plethora of recent work on perovskite structure oxides has led to exciting developments in the field of complex oxide heterostructures, correlated spinel structure oxides have garnered far less attention. In large part, this is directly related to the open spinel crystal structure with multiple vacant cation sites, making atomically precise growth of spinel structure films and heterostructures very difficult. We have studied the uniquely interfacial phenomenon of exchange bias in single-phase spinel ferrite thin films that are not biased by any distinct external layers. We have developed a robust, room-temperature synthesis method of spinel ferrites on technologically viable substrates such as silicon and amorphous glass. In addition to exchange bias, we have discovered unconventional magnetization behavior of the initial magnetization curve that persists with varying degrees of film crystallinity.
Pulsed laser deposition was utilized to grow (Mn,Zn,Fe)₃O₄ thin films on glass, (001) silicon, (110) and (100) MgAl₂O₄, and (110) and (100) MgO substrates. We tuned the crystallinity of these films from Δomega;~0.5-4.7° (as measured by x-ray diffraction rocking curves) through substrate selection, growth temperature and growth pressure. We achieved highly textured, nanocrystalline films that only exhibit the hh0 type reflections on silicon and glass, even when depositing at room-temperature. Transmission electron microscopy studies of samples grown on silicon showed that the pronounced texturing only exists in the out-of-plane direction. On samples grown on MgAl₂O₄ and MgO substrates, we stabilized epitaxial films of (Mn,Zn,Fe)₃O₄ at room-temperature.
SQUID magnetometry measurements showed that all (Mn,Zn,Fe)₃O₄ films grown at room-temperature were exchange biased, while epitaxial films grown at high temperatures had symmetric hysteresis loops about the origin. Depending on both substrate choice and crystallinity, the exchange field ranged from -20 to -400 Oe when field-cooled in 50 kOe to 10 K. Exchange bias in all films vanished by ~150 K. Unexpectedly, even when films are cooled in zero field, they are exchange biased at low temperatures. The initial magnetization curve also behaves anomalously in that it exits the magnetic hysteresis loop. This intriguing behavior is found in our films at all measurement temperatures (10-360 K), even in epitaxial (Mn,Zn,Fe)₃O₄ films which do not exhibit exchange bias.
The behavior of room temperature grown films can be understood in terms of structurally ordered, ferrimagnetic crystals which are embedded in a highly disordered, magnetically frustrated matrix. These disordered regions bias and pin the magnetization of the more ordered regions, leading to exchange bias and atypical irreversiblities in the initial magnetization curve.